I think I’ve found something interesting that bears mentioning. . .
While I hate to let plex off the hook for this, I think I have determined that this is an issue with the exoplayer (by Google) embedded in the Plex client rather than a problem with the Plex client or code itself.
Why? Well, I tried jellyfin to work around this issue, and found that the exact same issue at the exact same place occurred in The Martian (and some other movies I have). However, jellyfin allows you to change the embedded player from exoplayer to libVLC. . . and upon trying libVLC, the problem disappears.
So, since both jellyfin and plex use exoplayer, and both are affected (yet jellyfin ceases to be affected when exoplayer is disabled), I believe that indicates Google’s exoplayer is likely the culprit.
So. . . I’ve opened a ticket over at explayer’s github: MKVs from seamless branching blu-rays cause atmos audio drop outs and/or sync loss · Issue #10520 · google/ExoPlayer · GitHub
Kind Plex Folks, please help us bring attention to this issue over at exoplayer/google. Your involvement would surely be invaluable in getting this resolved!
–H