It's not a bug, RimWorld removes child body parts when you add bionics. So technically your pawn doesn't have any fingers to put a ring on. If you were to add a bionic arm, then your pawn wouldn't have a hand or fingers.
There are some mods that re-add those bodyparts when you use bionics. Now you have a better reason to remain a purist.

If you have a Druid in your colony, you can use the Regrow Limb master spell to regrow the entire arm, it comes with fingers.