After some experiments, it turned out that by simply filling in the tp_methods slot, the problem went away.

Still, the change to use _PyObject_LookupSpecial() appears to have missed the (older) use case where you don't define tp_members, but instead implement method lookup as part of the tp_getattr slot.

Not sure whether this is worth fixing. I just filed this report so that others running into the same problem can find it.