I have the exact issue. Old iphone stuck with 1 minute to go for the transfer. Did not say transfer complete as it should. So I am concern that its not complete somehow. However the new iphone 12 pro seem to work ok. but of course I cannot begin to check everything to be sure its completely transferred.
What next? Is that a software glitch? Seems it has happened to 2 other person on this thread.
I am considering whether I should reset the new phone to factory original and restart the Quick Transfer process all over again?