Usually this comes down to a case of bad naming (as in “not in line with Plex’ naming schema for tv content”).
Would you mind sharing how those files are organized and named inside your library directory?
Ignore what might have accidentally worked.
Your best shot is to go with the naming schema from the support article linked above, e.g.
Downloaded
TV Shows <- the directory linked to your tv show library
series-name
Season 01
series-name - s01e01 - optional episode title.mkv
series-name - s01e02.ext
...
Season 02
series-name - s02e01.ext
series-name - s02e02 - optional episode title.mkv
...
no disc structures, no additional folders, no different names for season folders etc.