I run across this issue a number of times now and while not a critical issue it is a pain in the ass if it isn't caught quickly.
When restoring a Backup from 8.306 to ANY UTM version the Additional Addresses Tab under Interfaces don't come online even though they are enabled. The fix is simple enough, disabled all of the Additional Addresses and then re-enable them. Until this is done these Interfaces won't come back online. Reboots of the UTM don't fix this. Only disabling and re-enabling the Addresses. I haven't been able to test a UTM 9 to UTM 9 restore to see if this still happens.
This is definitely a minor bug.
When restoring a Backup from 8.306 to ANY UTM version the Additional Addresses Tab under Interfaces don't come online even though they are enabled. The fix is simple enough, disabled all of the Additional Addresses and then re-enable them. Until this is done these Interfaces won't come back online. Reboots of the UTM don't fix this. Only disabling and re-enabling the Addresses. I haven't been able to test a UTM 9 to UTM 9 restore to see if this still happens.
This is definitely a minor bug.