I just ran into this issue today. Here was the sequence of events:
While logged in (i know, a dumb move):
-Pre-ordered dragonflight
-Did not select a subscription after order was placed (thought it was an optional step)
-Character became a trial account while in game (no ability to trade, etc). Note that transmogs were showing at this time (including new ones)
-Selected subscription (option: will decide later)
-Logged out and back in to enable full account again
Old bug not directly related to the upgrade. In fact, it’s completely random. Store-bought transmogs will disappear unprovoked from your Appearances collection (and from your character if you were using them) and then, later, randomly reappear in the collection (but not back on your character).
The developers thought they had it solved but apparently they didn’t.
There is nothing that a player can do to fix or prevent, since it’s completely random. My advice is to avoid using those transmogs even when they’re available, because you’ll have to spend gold re-applying them every time they vanish and then reappear.
It’s a bit different in my case. The character with the transmog on a cloak, still has it. However, it is no longer available in the appearances to use again.
UPDATE: After server maintenance, the transmogs are showing in the collection again.