If the show is actually matched perfectly, I’d verify that my media storage is 100% reliable.
If Plex server does a library update and media storage is not fast enough to e.g. spin up sleeping disk drives, Plex server may mark some media as ‘unavailable’.
That combined with this preference:
Settings - Server - Library - ‘Empty trash automatically after every scan’
will lead to removing these temporarily unavailable media from the library - and with them all customized metadata.
If you are using external media storage or even a NAS or (gulp!) cloud storage, you definitely want that preference to be OFF
