Right to the point. A systemd timer script had a too strict Umask setting.
Are you invoking pacman in an unattended manner in a systemd timer? If yes, you probably have another problem. I suppose you mean calling `pacman -Sywu`. That puts your system into a potential partial upgrade scenario and makes your local package versions desynced from from the sync database contains.
If your goal is to download packages to cache, see the `checkupdates` command from package community/pacman-contrib⁽¹⁾. `checkupdates -d` is doing exactly what you want, but without leading to a partial upgrade. ____ ⁽¹⁾ https://archlinux.org/packages/community/x86_64/pacman-contrib/