This seems like a bug. It has never happened to me before and does not seem like intended behaviour.
I have my media mounted in a plex VM (ubuntu 18.04.5) and sometimes the mount fails and does not gracefully recover. A quick reboot of the VM fixes the problem and no fuss.
This time I noticed not because the programme I wanted to watch was missing the file, but the whole listing had been deleted from Plex.
Running a scan, finds all the missing shows, but they are no longer in the correct order as Plex believes I added them today and it thinks they are unplayed!
Server Version#: 1.22.2.4264
Player Version#: 1.29.1.1974-a65b3ef3
From the logs…
././Plex Media Scanner.1.log:Apr 01, 2021 14:21:07.670 [0x7fd17a88d780] WARN - Error scanning directory, we'll skip and continue: boost::filesystem::last_write_time: Transport endpoint is not connected: "/crypt/The Kettering Incident S01"