This transmog sets seems to be missing the vestements, which are the non-robe version of the chest piece. Other regalia sets of this same model (but different colors) do include that piece.
So I imagine it is just a bug as all other parts of the set are available.
Iām noticing the same thing.
Update:
I exited the game and logged back in. It is now in my appearances.
This worked, thank you! I was having the issue above, as well as with the Korthia rep version of this ensemble, as it only showed the vestements version, not the robe, until I logged out and in again.