System doesn’t recognize APFS volume as startup disk

I installed Monterey on an APFS volume (I’m using Mojave on a 2015 iMac)

Actually, I installed Catalina, on the volume, then upgraded to Monterey. It rebooted and ran fine.

I rebooted into Mojave, and received a message, in the Mojave Finder, about a disk incompatiblity.

On my desktop (I have all my volumes displayed on my desktop), I have two new volumes - the expected Monterey volume, named Monterey - data, and another named “Update"

I ran Disk Utility on all three volumes but Monterey does not appear as a startup volume.

Any info appreciated...