I'm trying to make an item that gives a bonus to healing equal to your proficiency bonus. I created the modifier "Bonus Spell Group Healing" but when I select the additional bonus types "Proficiency Bonus" nothing is added to that character's healing spells.

Am I missing a step or is this not how the additional bonus types work?

That Modifier is not currently activated for our homebrew use, it only works for the Devs when they make official stuff.

Sweet thanks!

Is there an alternative to do that?

Unfortunately no.