0.51 update & DNS issue

Upgraded 2 MIAB servers. One running Ubuntu 18.04.5 now has an IP6 issue in the status screen saying

:heavy_multiplication_x: Public DNS (nsd4) is running and available over IPv4 but is not accessible over IPv6 at xxxx.xxx etc. port 53.

I checked the status page, re-ran setup, and rebooted the box. Error remains.

Suggestions please?