There are currently 3 achievements which are stated to be unobtainable. The Xbox related achievement will not unlock when you complete the requirements for: Savior of Elsweyr, Big Score, and Wrothgar Adventurer. It has been stated that these will also remain locked even if you try to complete them in a second character. The ESO in game achievement will unlock stating that you met the requirements, but the Xbox achievement WILL NOT unlock.
I can say from personal experience that this is true for Big Score. Due to the glitch, I have not tried to complete the other ones as they take a significant amount of time to complete and I do not want them to be glitched out.