Good luck. I have a TCL Roku TV that was broken by the 10.5 update. The issue is very similar to yours. The 10.5 update broke the TV's ability to communicate with every soundbar I've attached to it. With one soundbar the sound would just randomly cut out completely and would not come back until I restarted everything. With the other soundbar it doesn't recognize that the soundbar is Atmos or DTS capable until I change all the HDMI settings, then change them back. Basically, like you, it won't put out ATMOS over ARC/eARC unless I go through all the HDMI and related Audio settings, change them to something else, then change them back. Then it will output Atmos until I turn it off, then I have to go through everything again.
TCL support is nothing but a bunch of idiots, unfortunately. I've tried to get them to fix it for two months now. They asked me to send them the Tracker ID more times than I can count. I sent it to them every time. They asked me to make a video of the problem. I made a video. They asked me to make another video showing the same thing. I did that too. I've sent them everything they asked me to send even though they kept asking for the same thing over and over.
They even sent a tech to my house who saw the problem and said he couldn't fix it. After the tech said he couldn't fix it they didn't know what else to do so I asked them to provide me with a way to downgrade to the previous firmware that worked fine. They refused. I then asked them to replace the TV with the same model but with the Google OS instead of the broken Roku OS. They refused to do that too.
I then let them know that if they couldn't fix the problem or offer me a working TV I'd like them to either contact Best Buy and coordinate a return after BB's 15 day return period, or if they couldn't do that, to refund me the purchase price and I'd return the TV to TCL directly.
I didn't get a reply so I've tried contacting them countless times since then and they have completely ignored me. I think I'm going on SIX unanswered emails over the last several weeks. I guess if they can't fix their broken TV their solution is to ignore it and hope the owner gives up. Well, I'm not one that does that without a fight.
I was very nice to them but now we're going on three months since I reported the problem to them. In the state I live, the law gives them 30 days to either fix the problem, replace the TV with a working one, or give me a refund for the purchase price.
In the meantime the TV is gathering dust because its internal speakers suck and it doesn't work properly with any soundbars.