The runtime mirror list is created from the custom mirrorlist if it exists or the default pool.
The custom mirror list is created from the default pool.
In theory, if a user runs pacman-mirrors -f before pacman -Syyu they should, in most cases, get a reliable repository. imho, it doesn’t make sense to run the systemd timer pamac-mirrorlist because the state of those runtime mirrors may change before you actually run the update.
The thing is also, now if you create a custom list with -c Germany for example, it will not generate a proper list: The status file (https://repo.manjaro.org/status.json) that is being used by pacman-mirrors is outdated. Same goes for the status page itself.
It has not been refreshed since the 6th of March. pacman-mirrors will only list those that were up-to-date for your branch at that point in time (unless they were all outdated, then you’d get the full list. e.g. France).
For Germany for example in the unstable branch you only see two mirrors being up-to date although in reality it is more than those…
I personally don’t care because I just use a single mirror and I know it’s up-to-date.
But that status stuff will sooner or later lead to some confusion for some Manjaro users.