This is a bug confirmed by many people. There have been countless bug reports submitted, and it has still not been acknowledged by ZOS. This has been an ongoing issue for at least three months.
For those unaware: If you take the Warrior mundus stone, you can get "stuck" with it. You can change to a different mundus, but when you zone it will revert back to the Warrior. I have been stuck with it for months now. It breaks my build. I am strongly focused on spell crit, and the Thief mundus would be ideal for me, but I can't switch to it.
Please fix this, ZOS, or at least acknowledge it, add it to "known issues", and tell me that a fix is coming at some point. I'm not usually one to complain, and I LOVE this game, but this is a pretty major issue that keeps me from building my character the way I want to.