https://services.sonarr.tv/v1/scenemapping does have an entry for this show, (87711) with season -1… this fits with the most recent scene releases being S18 for this show vs the latest season being known as S17 in the tvdb/tmdb/imdb etc.
I think Sonarr is assuming it’s most recent season number needs 1 adding to find releases, which would be accurate if Sonarr had the most recent season as S17, (which would lead to searches for S18, which would be correct today) … except somehow Sonarr has the most recent as S18, leading to searches for a non existent S19.
Feels like a bug where Sonarr has added 1 to every season within the UI, but also still adds 1 when searching. I tried a manual import to S18, and it did indeed place the file into S18, which Plex etc understandably didn’t understand.
None of this explains where S8 and S9 have gone within Sonarr.
Update… after manually importing S18x01 then renaming the file and season folder (so plex etc could understand it)… Sonarr now shows S17 as the most recent, AND season 8 and 9 are now present.
Either something got fixed somewhere, or an odd bug has just sunk beneath the waves