I don’t know the answer, but I will point out something the OP is aware of, but others may not. Like many artificial languages, Volapük underwent significant reform in 1931. I don’t know to what extent Volapük Nulik and Volapük Rigik are mutually intelligible, but if they aren’t, that’s just yet another instance of artificial language reform—and not that interesting to students of artificial languages. It’s the unplanned shifts in artificial languages, I’d argue, that are unexpected and interesting.