Problems playing 1080p MKV format tv show episodes?

In the most current release version as well as the 1.18 beta, I have been experiencing problems playing back 1080p tv show episodes on an Apple TV 4K. These files are playing back normally on desktop/laptop and with other programs (VLC, etc), but in the Plex app it appears to load the first frame only and then plays back just the audio.

In the 1.18 beta, I tried ‘jiggling the handle’ as it were, and tapping play/pause or skipping ahead a couple seconds, and I have been finding this to be successful around half the time. Other times, I have to delete the 1080p file and replace with a 720p or lower.

Is anybody else experiencing a problem like this, and by any chance is there a setting change I can make on Plex server to resolve the issue?

Any chance these are H265 files? (MKV is just a container, doesn’t say much about the codec)