I was wondering why my extra buttons stopped working, and apparently there are now basic logitech drivers in the kernel which breaks the workaround (mix of generic mouse and keyboard polling) to get the extra buttons working.
Just an update to this post:
The problem was nailed down to NOT the linux kernel as was previously thought. The issue was systemd.
The latest systemd fixes the problem so just do a pacman -Syu and you get your buttons back!