I have no clue as the config has not changed for the last 2 months and the last change was removal of the OnlyCountry setting.
As I understand it the issue occurs on x32 iso's. Something tells me that an old pacman-mirrors.conf is placed in /etc on iso-build.
I remember that when @artoo and I worked the api out together we had to ensure a certain structure was present which otherwise would make the build fail.
Short story is:
If pacman-mirrors is run with the `--prefix $install` argument it will ensure a working structure in `$install` by copying the hosts `pacman-mirrors.conf` to `$install`.
In which case - if that file is an old version of the config then - whoever is building the iso, has to verify that the host systems pacman-mirrors.conf is the current as shown ^^^^^ above.
Note on branch
What the branch is named on a fresh install does not matter since on the first run of pacman-mirrors it will detect it is running on i686 and prepend x32- to the branch name.
Note on config file
Please check if a .pacnew exist for pacman-mirrors.conf.
Just noted ^^ this. It confirms my thought. I think @philm is building the iso's - so Philip will you please confirm your pacman-mirrors.conf on the machine building the iso's?