Maybe this will help you understand why I’m posting here: IPv6 Rarely Available in SL - Support / Technical Support - World of Warcraft Forums (blizzard.com)
To help further, I’ve consistently had a solid home and world IPv6 connection since it was introduced and my ISP supported it. Since the release of Shadowlands (and this is for the wow product as this is a wow product bug forum) it’s been misbehaving.
This is, indeed, a bug with the current architecture and the setting in the game client.