9.11.3 -> NOERROR / 9.12.X -> SERVFAIL

Summary

SERVFAIL for some domains - for example:

www.eclipse.org
www.redcross.ca

after upgrading from 9.11.3 -> 9.12.0 / 9.12.1.-P2

Steps to reproduce

[same config in tests]
9.11.3 -> resolves / NOERROR
9.12.X -> does not resolve at all / SERVFAIL

Reading through the release notes I cannot spot
anything worth trying.

Logs (with trace 99) + configfiles + console output + dumpdb:

https://www.undermydesk.org/servfail/working-9-11-3/
https://www.undermydesk.org/servfail/servfail-9-12-0/