Usually a simple relog on the character that unlocked it or a client restart will make it appear. If not, it should hopefully appear overnight! Things might be behaving a bit weirder and happening slower than usual due to the servers being prepped for Shadowlands, so that could be the cause.
In my experiences the Hidden appearances especially can behave really oddly and can take a while to arrive on alt characters, but they should eventually come through - if they don’t, submit a ticket and see if Blizzard can give you a hand with it! But yeah do give it some time first. They’re often the best-looking appearances so I just think of it as they like to keep you waiting
Usually when you loot an item and it’s soulbound you get it’s appearance for transmog and it’s account wide but with artifacts it works bit differently because it’s one item with 24 appearances. Becasue of that there is a special system that adds their apperances to transmog. This system scans what appearances you have unlocked on the particual character and adds them to you transmog collectoin. The important thing is that this system works only one time after you log in. This is why when you unlock new apperance for your artifact it won’t be added to your transmog colleciton. You need to log out and log back on that same character that you just unlocked new weapon appearance in order to force that system to kick in and add that appearnce to your collection.
This works for all artifact appearances, not just hidden ones.