Not quite your issue , but have seen a couple of things.
I actually have a case where I tried to change one of the PSKs in a running network , and it would not let me. Support told me I had too many PSKs in one SSID (not WPN, but the normal iPSK solution, but below the max, so I dont understand that answer - I think I have around 20 keys on one SSID).
But I have actually seen previously when creating iPSKs in a WPN network using API that sometimes the API did not return a "wifiPersonalNetworkId" key. So that PSK did never get a proper config. I think it has been cleared up recently.
But your problem does sound strange (I dont have THAT many keys in my WPN network, but I will keep an eye out).