The following information would be helpful in diagnosing an issue you're having with the Cloudflare IPv6 gateway:
1. A traceroute with the command of traceroute6 yourdomain.com
Directions for traceroutes
Note: Please remember to put 6 at the end of the command. If you don't get a response back with the traceroute6 command, it means that you're not running over IPv6 and you should use the regular traceroute command (traceroute yourdomain.com).
2. A dig of your domain using the command dig yourdomain.com ANY
3. A ping of your site using the command ping6 yourdomain.com. If you're not on an IPv6 connection, however, the command won't work and you should use ping yourdomain.com.
Please open a support ticket with this information when IPv6 is enabled on the domain. You can then turn the IPv6 option off in your DNS app if the site is having issues resolving.