[LinuxBender]

Just anecdotally and perhaps unrelated to your issue, I have a Primary DNS server in Vultr and at times IPv4 times out, then IPv6. It hasn't been persistent enough for me to start troubleshooting it or setting up 3rd party monitoring but I may do that today if others are seeing odd behavior now. Perhaps together we could create a list of service endpoints to monitor each other using curl or dig maybe and find a pattern to it.

Something to play around with

    # TCP AXFR.
    kdig @2001:19f0:b001:e83:5400:4ff:fe72:e740 +nocookie +padding=64 +retry=0 +all -t axfr example.net
    dig @216.128.176.142 +nocookie +padding=64 +retry=0 +all -t axfr example.net

    # UDP TXT or whatever
    kdig @2001:19f0:b001:e83:5400:4ff:fe72:e740 +nocookie +padding=64 +retry=0 +all -t txt example.net
    dig @216.128.176.142 +nocookie +padding=64 +retry=0 +all -t txt example.net