I think this is a bug with Plex. Previously there was a Premium and a HiFi plan. The Premium plan did not allow FLAC playback. Now any paid plan has access to at least basic FLAC playback, but Plex may need to possibly be tweaked for everything to play correctly. If you go to your dashboard, does it show that FLAC or AAC is playing?
I created this thread recently: Is New TIDAL HiFi Plan Supported Yet?
Also see this: Free upgrade to TIDAL HiFi - Can't Upgrade in Plex
Is that the case for everything? If not, I know occasionally the AAC version is what appears in Plex. You can try searching for the album and see if you get multiple results. One of those is usually the “HiFi” version.
