It may be a typo on your end but it is Fn and Caps which you have to press, when I use it it stays on until I disengage it
Plus it works not only for the number but for all Fn + characters (like @, # ...) so it is more like an Fn lock.
It does revert back when I switch fields.
Otera no tonari ni mo oni wa iru.
(Devils can exist alongside a temple)