I can't test this right now, but I have a suspicion that you need to set the argtypes and restype properties of the ObjC method manually. The derivation from the protocols argument doesn't always work reliably if the protocol isn't used as part of Pythonista itself. I hope this makes sense.
I completely forgot about argtypes and restype honestly. It makes sense, I will keep trying to find out correct argtypes. Thanks for your suggestion. Please let me know once you test.