It simply happens that way, where a corrupted file or setting is on a single character, as characters do have individual files associated with it. Resetting your User Interface is the best first step to troubleshooting this kinds of issues.
I can answer that! I’ve had character-specific UI issues before.
Typically that happens due to a corrupt WTF file, which is the file that saves character-specific settings in addons. I once had an error that kept popping up constantly on only one character, and deleting the WTF file fixed all of that.
again, you’ll want to do a FULL ui reset, you need to delete/rename the WTF folder along with the Cache and Interface. Launch the game withOUT any addons.
If you mean the Shadowlands pre-purchase transmog set, we know there’s currently some holdup with it showing on the Armory.
If it isn’t showing in-game, that’s indicative of something a bit more egregious and I’d recommend a repair to see if that helps. Possibly a reinstall.