In Pamac preferences - uncheck the option to check for AUR updates.
In any case it has has nothing to do with your mirror pool but more the recurring misbehaving of the CDN aur.manjaor.org.
That is correct - but any rerun of pacman-mirrors will rewrite the mirrorlist.
If one is managing a pool of systems with different users - e.g. family on different locations - then it is necessary to take precautions and ensure the updates are fetched in a controlled environment - then it is not enough to just set the mirror in the mirrorlist.
@tracyanne
If any interest - I have a customized pacman-mirrors PKGBUILD which patches the sources to facilitate precisely such scenario.
The undocumented feature mentioned earlier has been created to support a use case with a large amount of Manjaro systems maintained by an IT department - to ensure a homogeneus pool of systems.