Some Sites Failing to Sign-in

I have an unusual configuration for my services. I use CloudFlare externally and avoid it internally. A complete explanation of how I do this can be found here.

Recently I have configured IPv6 completely on my setup. After completing this I have started to have issues with logging in and I believe it is something to do with Keycloak and how the servers are connecting to it.

The working servers are

  • Tandoor Recipes
  • PGAdmin4
  • BookStack
  • Paperless-ngx
  • Grafana

The failing servers are

  • wiki.js
  • Apache Guacamole

The issue seems to be something to do with my split DNS. If I remove split DNS, everything works fine. Would the issue be with Keycloak or is it possible both of those services are doing something weird with dns lookups?