It looks like the GC untracks the None-filled result tuple in untrack_tuples, and it's never re-tracked again. This can also happen if it's filled with atomic values on an early iteration and the GC visits it.

Perhaps a simple fix is to call _PyObject_GC_TRACK on the result tuple inside of each zip_next call if it's not tracked anymore?

if (!_PyObject_GC_IS_TRACKED(result)) {
    _PyObject_GC_TRACK(result);
}
_PyTuple_MaybeUntrack(result);  // Worth it?
return result;

Although I'm not sure how much of a win we're getting from the result tuple reuse here - maybe it's easier to just not do that.