It could be that pamac was not really terminated, but continued to run in the background. I have had this problem several times recently, always when major updates have been rolled out.
I had already posted this in the German forum.
https://forum.manjaro.org/t/kein-bootlaufwerk-mehr-nur-noch-uefi-firmware-settings/175816
At that time I had the same problem due to a hasty restart, as I was not aware at the time that pamac was still running in the background.
In this case, I now use tail -f /var/log/pacman.log to check whether pamac is still running. For me a practical workaround.