@Gertjan Thanks very much for the very detailed explanation. I'm not sure that I know exactly what caused the problem, but I think that I needed to set 'Check IP Mode" to "Always use the Check IP service". It was set to Automatic (Default), and for some reason I was seeing an error with the Check IP service in the logs. @stephenw10 Thanks for the reply. In this case it wasn't Cloudflare. It was something in the way I had Dynamic DNS Set up.

@JeGr said in DNS hostname for dynamic IPv6 address: Newer Hosts tend to use EUI-64 if implemented so are not "predictable" by their MAC address anymore Actually, all IPv6 addresses are EUI-64. The host part can be either MAC based, random number or other. With IPv6, the EUI-48 MAC address is converted to EUI-64 by inserting FFFE in the middle and inverting bit 7.