These are surprisingly buggy, speaking more specifically about the daggers for Rogue (Both Sin and Sub).
The appearances that dont allow Enchant Xmog, like Sin’s Balance of Power variant, bugs severely if you transmog into them while having a Enchant Xmog on.
Even though you shouldnt be able to do this and normally, the Enchant look would be removed, but because this fails to happen, you do get to Xmog the Enchant, but this also makes it impossible to remove the Enchant unless you change the dagger’s transmog tone one thats actually capable of using them.
Even saving a Xmog look with the artifacts and with the enchants off, it doesnt work, if you have enchant look on, they will force themselves into the artifact and become unremovable until you change the dagger’s look.
I also noticed a trend of the off-hand dagger often disappearing, in both the UI and the character select (and Warband) screen.