This may just have been a timing coincidence for a booking number change but…
I rebooked the first leg of a b2b so ended up with a new booking number. My medallion app showed the old booking and would no longer let me to even open the travel checklist, health info etc.
Before contacting medallion app support, I did all the things they would ask me to do (delete and reload app, try using booking number as login, etc.) to reduce time on the phone.
But the last thing I did seemed to have worked !
I logged out of the app and logged in to the web version of checkin. That version did show the new booking so I updated all the items that did not carry over from my last completed cruise (emergency return airport, vaccination info, ordered medallion, etc.) and logged out. As soon as I logged out, I got an email that my medallion order for the old booking number was cancelled and when I relogged in to the medallion app it showed the new booking number and entire b2b itinerary !
The emergency return airport used to cause me issues with checkin when checkin was on princess.com so who knows that may have been the charm.