Locking the phone via “Find My Device” actually works without having it being activated as a “device admin app”. I just tried to lock my phone from my PC browser and it worked flawlessly. (You can even type in a message and contact number that will be displayed on the special lockscreen on the remote locked phone). I didn’t try the remote factory reset, though, for obvious reasons.
So, it seems to me, that the firmware update added this functionality to “Find my device” without it having to be granted device administration rights - but they forgot to remove this setting altogether which now leads to the lockscreen bug when it’s activated.
But just to repeat myself:
“Find My Device” including remote lock functionality is absolutely working, even when it’s not set as “device admin app”. And once the device admin setting for “Find My Device” is turned off, fingerprint unlock works as it should as well.