I have noticed that the HP regeneration while using a shadow : robe with XXI - XXIX upgrade affects the time of hp regeneration.
It affects other items that has HP regeneration per second. It basically makes other items effect obsolete.
However, XX and XXX robes are not affected by this bug.
Its kind of disappointing because I had spent a lot of delicious Kupo nuts upgrading my robe just to have this kind of downgrade in my stats and build.
If you need more details about the bug feel free to reply.
I hope this will be fixed on the future updates.
IGN: Death Core