The wording of checkupdates is a little confusing.
$ pamac checkupdates -a Your system is up-to-date. Out of Date: protonmail-bridge 1.2.3-1 AUR
It turns out that protonmail-bridge is flagged as out-of-date in the AUR.
Maybe change the wording to include "Flagged" or similar so that our OS isn't in the super-position of being up-to-date and out-of-date at the same time.