TCL TV + Amp + AppleTV audio failure

Indeed this seems to be a hdmi cec issue. Some TV depending on their firmware can drop cec connection to the soundbar. 
Often a firmware update on the tv can resolve the problem. Sometimes changing audio settings can help. 
First of all I recommend turning of eARC support on the tv as the Sonos Amp just Support ARC at all. 
Dolby Atmos audio also could be turned off in this case and hdmi audio output can be set to passthrough/ bitstream with Dolby Digital 5.1 as audio format. Also turn off Atmos Audio in AppleTV settings. 

I had a similar problem with my Sony A80J and Beam 2. It didn’t happen often, but required rebooting the TV to get sound back. I haven’t had it happen yet with my new Beam Ultra on the same TV.

As a guess, this isn’t a Sonos issue, but one related to the implementation by the Tv’s manufacturer. As previously suggested, I’d be checking for software / firmware updates, although for both the TV and the Sonos. I’ve certainly not seen any issues commonly attributed to the Sonos Amp, but if the behavior continues, I would recommend that you submit a system diagnostic within 10 minutes of reproducing the issue, and call Sonos Support , at your leisure to discuss it. Do not factory reset any units. Also, don’t post the resulting diagnostic number here.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.