I have extensive knowledge and use of MPV player. I switched from MPC-HC (that i had all tweaked out with madvr and manual custom compiled codecs) on my Windows machines to MPV, and use it often on Linux too. After setting up my own .conf and tweaking things to my liking, I find it far better, higher quality, and far smoother and minimalist a player that I always wanted for my needs and hardware setup.
I also use MPV player app on my Pixel 3 and Shield as my main video player for local files. Mainly cause i can set custom .conf (to a point) and add a bit of sharpening to make the vids more crisp (since I use a computer monitor, and not a TV with standard image adjustments ability) {The default Android player backend, Exo i think it is?} {I also used MX Player Pro for some time on Android.}
Funny u mention MPV though, as I used to use Emby for many years, before only recently switched to Channels. (I think Plex also uses MPV) ?
Emby switched to MPV as their back-end player some time ago for their desktop software (Theater) and Android Mobile app and there was lots issues and tweaks that took a while to hammer out.
After I got my Shield a few months ago, Emby on ATV for Live TV was full of micro stutters/pauses on all channels for me (using HDHR Prime at that time), and on Weather Channel most especially the audio would go way out of sync within 2 min. This only happened on the Emby ATV app, and i spent a extensive amount of time going back and forth on their forums, logs and all with the Emby devs...many test builds,.only thing they could conclude, was something with how ffmpeg was processing things on my hardware, and they gave up trying to help. The same channels played fine in the Shield in every other app possible, including Channels DVR. Even when set Emby to use External Player, and i set the standalone MPV app to use, that played fine.
I mention all this just in case it may give some insight or any ideas for the devs to look at to help.
For me, Comedy Central and Discovery TVE would be the go to channels for the buffering and stream stopping issues I had previously. DiY and HGTV (any Discovery owned network) also did it alot.
But that was caused by "Failed to download file error 403" errors when certain commercials came on, and was labeled by devs a "regional issue" that did not affect everyone. It was fixed (for me and several others that reported in) in the next pre-release sever build soon after.
Assuming u have the most up to date sever build, your issues do appear to be unrelated to that though i can only surmise.
The only similar issues i have seen reported with out a solution are from UK users. Something about the UK tv data streams does not play well with Channels.
Hope this may help any bit.