If you are using Microsoft for your DNS servers you really should update to
a version that includes this fix from March 19, 2019 monthly updates.
• Addresses minor issues with unknown options (unknown OPT) in the
Extension Mechanisms for DNS (EDNS) for the Windows DNS Server role.
https://support.microsoft.com/en-nz/help/4489893/windows-8-1-update-kb4489893
Older versions have even more egregious misbehaviour, with really old versions
(pre-EDNS support) being flagged as dead by recursive servers as they do not
respond consistently to EDNS queries. They respond a single time then do not
respond at all for a period of time.
Mark