I'm not him, but for me it was MacOS. After the update was installed and the system rebooted it presented a dialog asking if I wanted to be able to use iCloud f

I'm not him, but for me it was MacOS. After the update was installed and the system rebooted it presented a dialog asking if I wanted to be able to use iCloud for recovery if I forgot my Mac login password. I let it set that up. Afterwards I wondered if it was just storing the recovery key I already had in iCloud or if it had generated a new recovery key and my saved one was invalid. I checked my recovery key ("sudo fdesetup validaterecovery") and it was no longer valid. A bit of Googling failed to turn up a way to get a copy of the recovery key that was in iCloud, and I decided I'd rather have a recovery key I store myself in case I need to recover when I cannot get online so I switched it back. Switching back is easy. You just turn off FileVault, then turn it back on and choose to manage the new recovery key yourself.