Does it stay like this on hdmi 3 / 4 after power cycling?
What happens if you change back the cable? And then back again?

I doubt it’s the cable because I’ve tried multiple cables and all had these problems.

Did you try a one way optical HDMI cable?

Leaving the optical HDMI cable installed and the Apple TV on HDMI 3, I power cycled everything. Tested for 30 mins (I know that isn’t long), but no issues. I power cycled everything again leaving everything off for 10 mins. It still worked when it powered back on.

I then power cycled again and changed to the original supplied Sonos HDMI cable between the TV and Beam Gen 2. The issue came back straight away. Power cycled again and the issue is still there.

I just put the optical cable back in and it seems to work again.

I’ll concede that it could be something else, but what I can say is that in my testing it consistently doesn’t work with the regular HDMI cable and consistently does work with the optical HDMI cable. I’ll keep the optical cable plugged in and the Apple TV on HDMI 3 and will update here in a few more days.

If anyone with this issue has a one way optical HDMI cable, please can you try and replicate my findings. Alternatively, they can be bought relatively cheaply on Amazon.

No change with my Kabeldirekt 5m optical on HDMI 3, didn’t need to power cycle the beam. My understanding is whilst the TMDS signal is converted actively to optical, the ARC/eARC signals are still copper wires (at least on the shorter runs). Maybe some optical cables do more than just pass through and/or have optical returns.

I read somewhere that some features are still bi-directional on optical HDMI but thought I’d try it anyway. I can’t explain why it is working for me and not you. Just to check - you put the cable between the TV and Beam right, with the TV at the source end of the cable? Not between the source and the TV. 

I had the source end of the optical cable on the beam, other end in TV HDMI2. So I swapped the ends over (source in TV HDMI2 other into the Beam) and so there was no Sonos video displayed on HDMI2 (as now the cable is in reverse) however eARC was functioning and the dropouts ceased.

...Until I rebooted the beam and there were mini dropouts every 5 seconds. Pulling the optical cable out the beam didn’t clear them, but rebooting the TV did, and the 59s drops didn’t return either.

Reverting eARC to the Sonos provided HDMI cable also reverted the 59s dropouts

I wonder what the minimum set of connected pins is required to make eARC work but not necessarily any other HDMI feature. What might the embedded optical cable transceiver chipsets be telling the TV and/or Sonos when connected in reverse?

Nice one. It is good that you could replicate this.

So whatever the Beam is sending to the TV to cause the drop outs is something that would go down the optical part. I guess that could narrow it down, but I’d guess Sonos already knows what the issue is. Either way, if this workaround works for the long haul, it is good enough for me.

I re-wired my sacrificial Amazon Basics “HDMI with ethernet cable”. Pins 1-12 (3xTMDS data and TMDS clock) are now “open circuit” and pins 13-19 (HEAC/eARC, DDC, HP, 5V) and shield are connected (I finally found my HDMI cable tester as per photo). Behaves the same as the reversed optical cable which means that the active components  (e.g. I2C buffers) inside the optical cable can be eliminated from contributing to the workaround.

My guess is that the TV is losing TMDS clock sync, although there is apparently some control data sent over the TMDS lines too. Either way, you’ve isolated the issue even further, and not only, as you say, should Sonos know the cause already, they could have offered a workaround months ago by defeating their TMDS signal (it’s still not enough evidence to attribute an actual bug to either/both the TV/Beam2).

So, @Corry P - feature request please: a switch to disable the B-roll video stream on the Beam 2 until a full fix is implemented. It’s not like it does anything useful anyway, a “now playing” screen would have been much better use of the video output.

Hi @RANDUSR23296 

I’m not sure a feature would be added to mitigate an issue that we’re not yet sure is for us to resolve, but that’s not for me to decide. To make a request, however, you’ll need to start a new topic thread with the request in it - please feel free to do so.

Looks like no fix in 15.2, disappointing as I hoped this would be the one. 

After 8 months, Sonos still don't know if it's even their bug...

That is disappointing. At least my workaround is still working (see my previous posts if you haven’t already), so I’m happy enough that it will no longer cause me issues. I’ll still follow this for a proper fix though.

For anyone interested, this is a the cable I’m using which fixed the problem for me. 
https://www.amazon.co.uk/dp/B09YPR5SD4

Dang unfortunately they don’t ship to Israel or it’s all out of stock, but Amazon says currently unavailable :(

Arguably, there's even less chance of seeing a change in the Beam 2 now there's a relatively cheap external workaround identified.

However, how does your reversed optical perform with Atmos streams? Mine keeps dropping them so I won't recommend my optical cable as a workaround. I may make up a more robust TMDS-less cable and see how that behaves on DD/MAT  

No issue with Atmos here, although one time audio just disappeared completely but I’m pretty sure the Apple TV just crashed (restart sorted it). 

Would be one thing if they can confidently proclaim it isn’t theirs. But to still be clueless after almost 9 months? Bravo. I’m not expecting anything from Sonos anymore. Just biding my time until the next upgrade.

Anyway HDMI1 still works for me and I turn off my power strip 2-3 times a week. I’ve only skimmed recent posts -- does a way one cable disable/downgrade HDMI-CEC? I’ve actually ran out of ports so I use an HDMI switcher on HDMI1 where I connect all devices that regularly use PCM 5.1. I guess that’s another option people may consider.

CEC still working. The only thing you seem to lose is the Sonos image when selecting HDMI 2 (which is pointless anyway). 

I ordered the cable. Should be delivered tomorrow. I will test it and report it here, if the dropouts are gone, too.

Make sure you run the cable with the TV at the source end. Fingers crossed this works for you too. I’ve been watching loads lately with 5.1 PCM and I’ve not had an issue. 

I’ve only been set up with Beam 2, One SL surrounds, and LG G2 55 for a couple of weeks - I’m new to the Sonos ecosystem, and just bought the G2 - but the dropouts playing on Switch with surround on had been driving me nuts. Hadn’t noticed the issue with anything else (streaming content on native apps, gaming on Xbox Series S) and wasn’t sure how to troubleshoot between Switch, LG and Sonos.

I added a Panasonic 4K BD player this week, and noticed the dropouts immediately on non-Atmos content… also with PCM 5.1! Now I knew what to Google, it wasn’t just a Nintendo issue, and found this thread. Stayed up until 2am a couple of nights back reading the whole thing, with mounting dread at the lack of progress re. a proper fix… what a saga. However got to your input at the end (perfect timing!) and have the cable you linked due for delivery today. Massive kudos and thanks to you and everyone on this thread for all the troubleshooting you’ve been through, if I’d had 9 months of this I think I would have given up.

I’ll let you know how I get on once the cable arrives, fingers crossed!

The cable arrived today. After a first short test, I can say that the Nintendo Switch no longer has any dropouts. Will check the PS5 later today.
I'll report back here in a few days if everything is still running stable!

Very positive news! Hope it continues to do the job. Amazon c*cked up my delivery, so still waiting on mine

Thank you very much for your testing and also thanks to @RANDUSR23296 for the extensive debugging work!

I’ve gone ahead and ordered said cable as well (from Amazon Germany) and will report back once I was able to test it.

I bought the cable from amazon singapore and will be testing as well.

Also bought the cable and waiting for it to be delivered. I am following this thread from the beginning but haven't had the patience to do the extensive testing and troubleshooting @RANDUSR23296 or @MJW75 have done. Many thanks for your efforts.

Replacement cable arrived today… further testing required, but played an hour of Switch with Surround on, and played 30 mins of a 5.1 Blu Ray that was problematic before… zero dropouts thus far!

Many thanks again to @MJW75  and @RANDUSR23296