I’ve got a transmog on my character and the item slot is NOT transmogging. I’ve even tried taking the item off entirely and the screwed up appearance is not removing. I’ve tried changing the transmog on the slot and it’s not applying the actual item transmog I’m using.
When a slot refuses to transmog that usually indicates you could have by mistake equipped a piece of armor inappropriate to your class (in your case cloth instead of leather). Maybe if you were more specific someone might come in and confirm it isn’t an isolated incident, to help narrow down a bug.
Otherwise anything is possible because the patch has a lot of other problems.
My helm transmog, which has been Crown of the Icy Blue Rose ever since the crown was released, keeps appearing as something completely random. It’s not even the default look of the item itself, it’s something I’ve never seen before and have no idea what it is
What I can see is that the item is tagged as general cosmetic piece rather than specific armor type.
I myself am using a cosmetic explorer’s hat and have tried other appearances of similar classification (I don’t have your helmet though) and it all worked as intended.
Perhaps it is in some way related to the bug disallowing artifact transmogs that don’t explicitly match an equipped weapon. Who knows. Sorry for not being of too much help.