9.16.6 insist failure

Received by security-officer:

Hi,

I just upgraded the four nodes in our anycast resolver cluster to 9.16.6. However, shortly after starting, one of them decided to exit, and in the log I find:

Aug 21 14:00:26 res named[20987]: resolver.c:5125: INSIST(dns_name_issubdomain(&fctx->name, &fctx->domain)) failed, back trace
Aug 21 14:00:26 res named[20987]: #0 0x41f368 in ??
Aug 21 14:00:26 res named[20987]: #1 0x7a49a5e168dd in ??
Aug 21 14:00:26 res named[20987]: #2 0x7a49a72fa0c7 in ??
Aug 21 14:00:26 res named[20987]: #3 0x7a49a72fbdd1 in ??
Aug 21 14:00:26 res named[20987]: #4 0x7a49a7300818 in ??
Aug 21 14:00:26 res named[20987]: #5 0x7a49a73048a8 in ??
Aug 21 14:00:26 res named[20987]: #6 0x7a49a7305395 in ??
Aug 21 14:00:26 res named[20987]: #7 0x7a49a7306831 in ??
Aug 21 14:00:26 res named[20987]: #8 0x7a49a5e3a317 in ??
Aug 21 14:00:26 res named[20987]: #9 0x7a49a340c1d8 in ??
Aug 21 14:00:26 res named[20987]: #10 0x7a49a2e87af0 in ??
Aug 21 14:00:26 res named[20987]: exiting (due to assertion failure)

this instance was started some minutes earlier:

Aug 21 13:48:31 res named[20987]: starting BIND 9.16.6 (Stable Release) id:25846cf

I wonder if this is related to an incomplete fix (?) of CVE-2020-8621; this name server is doing forwarding via

options {
        forwarders {
                [redacted];
                [redacted];
        };
        forward first;
};

It ran with "qname-minimization relaxed;" at the time (explicitly configured), I have for now changed it to "off".