Try to do an “analyze” on the file. If Plex scans the file mid-copy, Plex might see the empty 0 Byte file that is usually created by the file system on copy and fill all kinds of bad info. Analyze will tell Plex to re-“analyze” the file attributes (codec, length, subtitles, etc).
If that fails to fix it, make sure to do a full Plex Dance on the file itself (in particular, after you remove the file, then do “Empty Trash”).
