Is the issue tied to a location or specific speaker?

Specific speaker. I’ve swapped its location with the working Sonos One SL, and moved it closer to the router. Going as far a hard wiring to LAN. Same exact issue happened, temporarily losing connection to eventually come back, to lose it again.It is this one specific Sonos One SL

Likely this is a hardware issue. Have you submitted a diagnostic immediately after an event, then worked with SONOS phone support?  Don’t Factory Reset until you’ve worked with phone support because the Factory Reset will destroy diagnostic data.

Note that all traffick for a Group passes through the player used to start the Group. Similarly, all traffic for a pair passes through the left speaker.

If it lost connect whilst Ethernet cable-connected it doesn’t sound good. Next time it happens, generate a diagnostics report and contact Sonos support. They may see things in the report to isolate the issue. 
 

Edited to add:  ​@buzz was posting whilst I was typing

Diagnostic sent several times. The device is almost 4 years old, so doubtful that I get a replacement from Sonos. Thanks for the input and suggestions. 
 

The left speaker keeps its connection even after the right has dropped and the app drops the room entirely. That left speaker keeps going and going. 
 

Remember, Sonos doesn’t respond to diagnostics sent, unless you call Sonos Support directly to discuss it.

I’d imagine they get way too many false positives to do so effectively. 

I’ve had multiple conversations with their technical support team. Phone support, email and social media. 

Currently there’s little I can do that hasn’t been done in regards to troubleshooting. 
But weeks of 60 plus minute support calls, is a futile endeavor. 

Sorry to hear about your issues with Sonos One SL . The issue is most likely a failing network interface card (NIC). Unfortunately, there is no path to replace or repair the NIC.

Not to add more bad news; but the upgrade path for the Sonos One SL to an Era 100 is and is not feasible; as you would still need another Era 100 to create a stereo pair. However, if you are willing to forego creating a stereo pair you probably have an upgrade credit to use towards the purchase of an Era 100. The credits usually garner at least 15% discount. Click here for Upgrade Info.

You can also check eBay for a deal on a new, used or refurbished Sonos One SL at this link. 

Or, or a replacement refurbished Sonos One? That’s a fair trade in at this point. Keeping them from stereo pairing has extended the connection for the problem speaker for a few minutes longer. 

I’ve been testing using the Sonos app for content this whole time. Just started testing over AirPlay directly to the one speaker, to rule out any problems that the app may be introducing. 

Eureka! I think I isolated the problem, thank Goodness. Streaming directly from iPad to the Sonos One, and not in stereo mode and it’s been going strong for 18 mins (a record). I might get froggy and connect the Sub 3 if it’s still going for another 30 minutes to try and break it. 

Update: 30 mins and no drops or disconnects. Makes a person think  

This is fascinating, attempted to connect Sub3 from the Arc to our problem child. I got the audio cue of a successful pairing (very muted btw) but the sub is not connected to anything. Several attempts to connect have been unsuccessful. 
 

update: reconnected the Sub back to the living room Arc without issue. Audio cue was as normal. 

Hmmm….not to be critical or rain on your parade but I can’t quite understand why streaming in mono would make any difference vs stereo. Unless you saying that you’re not streaming to the speakers  as a stereo pair. However, even then I still believe something else is a foot with the speaker.

But...if it continues to work who am I to argue with the Sonos gods 

Stereo pairing seems to break it. Pairing a sub is just unsuccessful with no ill effects to the connection. 

Obviously our posts crossed 

If you removed the sub from the Arc it should have given you the option to move it to another room. If not then it wasn’t successfully removed from the Arc. At the very least it should have appeared in the app as “unassigned”. Although my “gut” still suggests it’s  still a failing NIC. The Sonos One SL needs to communicate with the Sub via its proprietary 5Ghz band.

The sub eventually was labeled unassigned after several failed attempts to connect to Sonos One. Reconnected back to the Arc without issue and quickly btw. 🧐

FYI my router is set to SON, otherwise it breaks the connection to other devices ie: AppleTV PS5

this was never an issue before re: Sonos OneSL

Another new quirk found! Removing the sub from the Arc prior allowed the Sub connect to the Sonos One…however; the streaming connection dropped immediately after a hour of playing uninterrupted. 
 

update: was able to establish a connection via airplay to  the sub and single Sonos one pairing. Stopwatch started

Here’s my assessment after several hours of testing and troubleshooting: While logic points to NIC failure, I believe with little doubt that the issue is pairing speakers to either a room or to each other directly. Streaming directly from the Sonos app is wonky as well. But the main issue is the speaker pairing. I’m not about to test with my perfectly working living room setup. But with complete confidence, it’s a software issue not hardware. Sidebar; a single Sonos One paired with the Sub 3 sounded amazing  

Test pairing of “troubled” Sonos One and Sub 3 are 30 mins and going strong.  So not the speaker and not the wifi. IMO, it points to only a few scenarios that can be the cause. Especially when this system has worked for years until very recently.