I found a new way that mine will lock up... If I ask Bixby to open Secure Folder, it will lock up on me, and it was the first time entering secure folder since my last reboot. (I reboot every morning, just because.)
Still, my issue "seems" to only happen when going into Secure Folder for the first time after a reboot, *if* I access it using a notification, and now additionally I've discovered it will lockup if I ask Bixby to open Secure Folder.
(Going in manually has still not, for me, made it lockup.)
I think I'll have a go with Samsung support today... They're just going to tell me to do a factory data reset, but... at least they'll know.