The flipped side of Primal Amulet, Primal Wellspring, doesn’t really function correctly at all.
Used in a mono-white Elspeth deck to combo using Approach of the Second Sun.
Flipped the amulet to Primal Wellspring and it was consistently doing these actions
Turning 2 gems to Green or Red. It might have done Blue or Black on occasion, but it was never any White gems.
Returning the spell to my hand with 0 mana. Anytime I played a spell costing more than 9 it bounced it back to my hand as expected, then showed +6 (maybe 5?) going into my hand, showed the spell half charged, then dumped all of the mana on the spell to 0.
This happened a dozen or so times before I realized it was 100% bugged. I double checked Approach wording and Primal Wellspring.
This was Nicol Bolas in Return of the God-Pharaoh.
It may be bugged only against him, I did not continue to test it afterwards, considering it was somewhat pointless. The spells were never landing in my graveyard so they did not ramp off themselves while it was out.
Every single card that converts gems to “your planeswalker’s colors” has been bugged since Ixalan came out (they convert to your opponent’s colors instead).
This includes the second node of Across Ixalan.
Most of them have been fixed by now, but since it appears to happen on a case-by-case basis they must have missed one.
I also found Amulet//Wellspring to not work properly, but I didn’t like it much anyway so I never really bothered testing it out