Focus regen on mage rune

So in infused my mage staff with a chipped topaz (please don’t spam “my god u have a chipped topaz” “gosh where did u find it”). Focus regen-on damage working as normal on auto att but seems doesn’t work on rune (ie Fireball). I ve also run a bow focus build and works on arrow runes.

is it bugged or is how supposed to be?