The bug is caused by the IOS 11..15.1 updates. It is documented, bug reports are filed, and every available version of AE will crash immediately if you try and propagate Rotobrush. We will have to wait for an update. It may take a while. Premiere Pro, Audition, and a bunch of other Adobe apps are already Universal (M1 compliant) but After Effects, even AE Beta is still a universal app that needs to run under Rosetta.
Most of the time, before I run an IOS update or a Windows update, I make a bootable copy of my boot drive co I can roll back in a couple of minutes if there are problems. This was one time I got caught so my M1 Mac is useless for Rotobrush until an update fixes the bug caused by Apple "improving" security. It's a killed background process that causes the crash.