the package is not there and so the AUR grabs it from the AUR as well. Perhaps it is even the Git version with an unclear version number

You will see that the aur package will use a git version and you will also be asked to remove the conflicting package when you are installing a git version.

And once again, this isn't unique to manjaro, on my arch install yuzu broke because they were using dynarmic from the aur instead of using the one provided by yuzu itself.

Also gimp and gegl are already on both the arch and manjaro official repos, If you are using git packages and you don't update them lots of things will break regardless if you are on any arch distro.

Now I wonder if pamac checks for updates of git packages by default, because your git packages will not be updated unless you explicitly tell yay to do so (yay --devel) I think paru every does it automatically with every update but then again most people will use yay instead.

Suffice it to say, when I used Manjaro, I got the impression that the AUR broke all the time and that using the AUR broke my install from time to time. Now that I use Arch, I do not have those issues and I realize that it was Manjaro all along.

My experience has been quite the opposite, a few months ago my install broke to the point that I could not update the system, turns out it was because of the arch migration and my system wasn't incorporating the new pacman.conf.new.