I am pretty sure this is a totally different issue from the above.
I’d disable the DLNA server in Plex,
Restart the Plex server
Add some new media
Wait a few days.
If it doesn’t happen to those new items anymore, you can be sure that it was caused by a DLNA compatible device or a software in your house.
Some of these are building a media library of their own. For this they play the videos for a short time, to create a screenshot.
Either find the responsible device and disable its “library creation” activity
Or
Disable “timeline reporting” in the DLNA preferences of Plex (which causes playback activity by DLNA devices to get ignored)
Or
Disable DLNA totally
