To preface, this isn’t the first time this bug has happened and is reoccurring on this specific character. The transmogs for the Legion Artifact weapons (and their respective appearances/colors) have been bugged, not showing up or only partially showing up. This is unique to this character and is not account-wide. In addition, this character received a level-boost from the token, which may have been (partially) responsible for the bug. I have done a hard reset on the UI and have uninstalled and reinstalled the entire game, neither have fixed the bug. The only partial fix came from a previous GM helping, which is linked below:
It’s sort of been broken since the end of Legion and going into BfA as well, where you cant transmog a different off hand with your artifact main hand, it will by default transmog the artifact off hand over the look you’re trying to transmog. You even spend the gold thinking you’re transmoging lets say Aegis of Argus over your shield along with Oathkeeper, but instead nothing changes and you’re still stuck with that artifact shield. And its vice versa too with main hand.
I know by default if you transmog your main hand with an artifact skin, then the off hand automatically will take on its accompanying partner as well from the model duo back in Legion. I’m sorry i vented more then actually give you a solution, but yea artifact transmoging has been broken for awhile.