Comment 6 for bug 1917099

I will have some time to demonstrate the problem on a couple PowerDNS servers tomorrow, but this is the closest I could get to finding it described in RFCs:
https://tools.ietf.org/html/rfc5936#section-3.2
https://tools.ietf.org/html/rfc1034#section-4.2.1

>The RRs that describe cuts around the bottom of the zone are NS RRs that
name the servers for the subzones.

>One of the goals of the zone structure is that any zone have all the
data required to set up communications with the name servers for any
subzones. That is, parent zones have all the information needed to
access servers for their children zones. The NS RRs that name the
servers for subzones are often not enough for this task since they name
the servers, but do not give their addresses.

The problem is that the nameserver being queried doesn't serve both client and parent zone.. if it only serves the parent zone then the nameserver doesn't know where to send the queries if you are relying on recursion as described in the RFCs.