Turned out that moving the libalpm hooks from specific packages to
mkinitcpio had an unexpected side effect: Updating these packages does
no longer trigger the rebuild of initcpios. This is beeing worked on
upstream, but let's push an interim workaround for now to be ready for
the next systemd feature release.

https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio/-/merge_requests/401