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.
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.
Even pamac can do it - it does not indicate it is a good idea to automate rebuild of AUR packages.
Let’s me emphasize - changing your mirror pool to a locally maintainedl mirror does not in any way affect how Pamac rebuilds AUR packages.
IMO opinion AUR packages is best maintained manually. Given the somewhat unstable mirror of the AUR database maintained by Manjaro - the manual check makes even more sense.
You can use yay to check if updates are available for your installed custom packages.
yay -Qua
You could create a simple script using inotify to show a message if theres updates to your custom packages - then use systemd to run the script as your user.