Mysteriously sometime between 2 (1 hour after I posted, I gave up) and 16 hours (now) - the device mysteriously appeared, but without a device owner - but that's easy to fix.
Supposed to be able to use activation email for multiple windows/mac devices (not android/iOS)
Event
5:55 pm10/17/2022
Device "MACBOOK-64AEAC" has been automatically moved according to rule "AddMDMDevicesMoveFromUnassignedGroupRule" from group "local" to group "Managed devices".
localAdministration Server
5:36 pm10/17/2022
Device "MACMINI-6D2C2B" has been automatically moved according to rule "AddMDMDevicesMoveFromUnassignedGroupRule" from group "local" to group "Managed devices".
localAdministration Server
but only one of the two new devices shows up under users/devices:
Status
OS
Name
Device owner
Group name
Security profile
MACMINI-6D2C2B
Mac OS X
jeff
*****@*****.tld
Default