While doing some DNS changes for a hostname with quite some DNS records I was encountered with some very strange behavior in which certain users were receiving different DNS records than other users. Even though I made changes to a certain A record, a specific group of users would never receive these changes, and it would always be the same group.

After asking one of these users to send out the details of a ping command, I found out that the host was being resolved to an ipv6 address, that looks like 2a00:1450:400c:c05::71. After inspecting the DNS records once more, I found out that there was a lingering AAAA record that still pointed to an old ipv6 address. Each of the users with a ipv6 internet connection were automatically being served the AAAA ipv6 DNS records. After removing the AAAA record, the DNS automatically resolved using a fallback to the ipv4 A record and everything went back to normal.

If you want the AAAA records to point to an ipv4 host, you can use tools like ultratools to convert you regular ipv4 address to an ipv6 compatible address.