I have the same exact issue, can confirm it is NOT the source video. I have multiple video files that play fine directly on PC or Mac, but when played through Plex running on a Synology 218+, they all encounter pixelation issues like the ones pictured above. My Synology worked fine with the same files up until earlier this year, and for months my drive has been pretty much useless since all files seem to be doing this now.
Doing research online it seems like I am not the only one having this issue. Some users found resolve by editing the Preferences.xml file, however this did not fix my problem.