[ISC-support #20639] Problems arising from leading underscores used in QNAME minimization
https://support.isc.org/Ticket/Display.html?id=20639 (This ticket also involves some odd behavior by the authoritative servers, which is not pertinent to this issue.)
The present method of determining the zone cuts using queries containing a leading underscore, could be improved upon by using NS queries instead.
Per Mark A:
I suspect this is an inherent side effect of using '_' prefix labels. We don't get a negative cache entry for the intermediate node to know that we don't need to query for it. Going back to direct NS queries would be the way to fix this by looking for the lack of a non existance for the NS RRset in the cache before making the probe.