In case this has potential benefit;
Posts in another thread suggest that switching Pamac from pamac-gtk3 to pamac-gtk (or vice versa) might solve the apparent disappearing authentication window problem.
This would be achieved by either:
sudo pacman -Syu pamac-gtk
…or…
sudo pacman -Syu pamac-gtk3
depending on which is currently installed.
You will be asked to confirm replacement, either way.