First I thought it was just a fluke, but I've now had this happen twice in eight days.
My pi completely and instantly locks up when updating / having updated sometimes. This happened twice now when updating the bootloader and/or kernel files, so afterwards the pi wouldn't come back up since the needed files for proper booting were 0 bytes, since the filesystem hasn't had time to flush buffers properly before the issue occurs.
Putting this here so maybe others can chime in whether this has been an issue for them recently.
My boot partition is vfat on an SSD. Is it possible the 5.15 vfat driver has issues? This never happened on the 5.10 branch.