Looks like there may be a solution: make sure the email address you use in Tidal is the same one you use for Plex.
See TIDAL: Subscribed but not linked
Changing the address did the trick for me. I now have my “old” Tidal email that I still use as the username (in the Tidal apps), but in my account setting I changed the email address to the one I use in Plex and tadaaa… it worked.
Edit: meanwhile Plex has issued a fix 
