Upgrade to 20.7.1 fails with unbootable device

[KoS]

I have upgraded a box via the GUI to 20.7.1 (from 20.7). The upgrade seems to finish fine (see upgrade.png), but when i reboot the device I get a "not a bootable disk" error (see unbootable.png).

Any idea what could have went wrong? FYI: The device is using ZFS (installed via FreeBSD 12.1 plain installation + opnsense-bootstrap) and it is an APU board. I didn't expect that the upgrade touches the boot code in any way. FYI: the upgrade from 20.1 to 20.7 failed the same way, but I had thought that was some other issues and reinstalled it from scratch. (Just the ZFS install is a bit a pain as it takes extra steps to first install freebsd and then the bootstrap...all via serial console).

greets
KoS