The #1 reason for this happening is your files are unable to be matched, remain unmatched, or are forced unmatched when Plex prefers the bogus embedded title field in an MP4/M4V file - over a correct and proper file name - if you have any of those.
Plex scans the library, finds or creates unmatched items, goes looking for them, blowing all your edits. Yea, that lock does nothing 'cause Plex is hell bent to get that item matched. Read on.
If the item can’t be matched at all - home movies, etc:
Fix Match/Search Options/Agent Dropdown/Personal Media
Select the only item in the list - yours - and from that moment on Plex won’t try to match it again. You can edit safely.
If you have crappy file names - fix your file names and get a match. Or Fix Match.
If you have MP4/M4V files - Do the LMA Hack:
That will stop Plex from peering inside your MP4/M4V files to make your life miserable. Combine that with some decent file names and all is well.
Battle Plex over a file name - and you get what you get.
