As we previously explained, the bug is tied to the area where you visit four spas in the Shangri-Spa area in order to gain access to a secret fifth spa:
The issue is that once the NPC guarding the entrance asks to see the membership card required to enter, it is removed from your inventory entirely – so if you exit the area for whatever reason, you can no longer gain access because you don't have the membership card, nor does the NPC remember taking it from you; it's also not possible to obtain a replacement.
There's no need to worry, though - as Nintendo has heard about the issue and is working on an immediate fix. Polygon got confirmation of this from an official representative of the company:
"We are aware of reports about issues affecting some players of Paper Mario: The Origami King. We are working to resolve these issues and plan to address them in a software update. We apologize for any inconvenience."
There's no exact word on when this software update will arrive, but given Nintendo's recent response time to glitches in Animal Crossing: New Horizons and various other first-party titles, we shouldn't have to wait too long at all.
Have you encountered this game-breaking bug in Paper Mario yourself? Leave a comment down below.