I had a similar weird experience last night.
I've been using ST4 Beta on multiple iPads since day 1 of beta.
I did the Testflight 'purchase' when that rolled out the other week.
I had spent a few solid days with my iPads at home setting up Network Session related local lyrics, testing sync etc.
I got to a technical rehearsal last night, and on my Host iPad, as soon as I tried to start Hosting a Network Session, I got the 'paywall notice'. I had literally been using it a few hours earlier at home. No new build. Maybe I had rebooted the iPad (I commonly do this before heading to a rehearsal so they all start 'fresh') which may have been the trigger for this behaviour..
A few seconds of "WTF!?!?" annoyance... then pressed on "Upgrade from Stage Traxx 3" and it all went back to normal.