Like you I'm using output for simple stereo through a soundbar. I've not noticed the distortion but it's probable that there is some. TBH don't want to mess around with this too much, I just want to watch the TV and am not going to waste time making support calls with TCL which to me seems like a hardware issue that they won't acknowledge. I am disappointed, but now I know better and at least it mostly works
I have been struggling with this issue for several years since I purchased a Tcl model 65s405. Tcl's customer service had me switch to a different audio setting which initially fixed the issue. But sometimes when a new software update comes out the problem comes back. The setting I am using now is the "auto passthrough" setting and it appears to be working, but I won't be surprised if the problem comes back again after a future software update. I noticed that my dropouts occurred during abrupt changes in the video picture even sometimes when streaming with roku, but mostly with my cable box hdmi output.
Same problem here. I noticed that it does NOT happen on the internal speakers; only on Optical Out, regardless of the sound output settting.