Both packages provide /usr/share/libalpm/hooks/80-atd.hook
Thanks for reporting this, this will be looked into.
at-openrc also provide 80-atd.hook.
All
at-<insert_init_system> packages provide it. It is just that the package
at didn't provide that hook until recently. As said above, this will be looked into. Patience.
at 3.2.4-1.1 should fix this.