I ran into the same exact issue with a fresh 12.x install. I believe there is a bug in the default master.conf file for Postfix.
This line (near the top):
pickup unix n - - 60 1 pickup
Should read:
pickup fifo n - - 60 1 pickup
And the line 2 lines below:
qmgr unix n - n 300 1 qmgr...