It seems the Magni's cap set (Magni's Cap - Odin's Blessing - Stone Buckler) set doesn't give the bonus if the cap is in a custom helm.
If I'm wrong feel free to correct me.
Magni's Cap Set
Re: Magni's Cap Set
I'll take care of that right now. It should be fixed within the hour.
EDIT: Fixed within 5 minutes.
EDIT: Fixed within 5 minutes.