I didn't find any posts about this, so I thought I would add what I found.
After a couple years of working flawlessly, after some update or another I was getting my movies added and their names set as the DVD name, sometimes even as "DVD_VIDEO". It was getting metadate and posters just fine, but then changing the name back to all CAPS and abbreviated like the DVD name.
I tracked it down to Handbrake setting a tag for 'Title' on the file, and by default setting it to the DVD name. On my plex server under Settings > Agents > Movies > [Personal Media] [Freebase] [The Movie Database] I uncheked the box for "Local Media Assets" and then rescanned and everything was back to normal.
I had never played with the agents before, and I'm not sure if tags used to be a part of handbrake, so I can't say where the change happened that caused this issue to pop up.
Hope this helps someone