This is not a new issue, but it was at least alleviated in 9.0 (likely indirectly, else it would’ve been fixed completely)
The following items:
Renowned Explorer’s Mantle
Renowned Explorer’s Lucky Bangle
Illidari Blindfold (&other IDs)
Vulpera Heritage Footwraps
(and likely several more)
are items without an appearance, and are thus considered by the game to be lookalikes of the “Hidden” items used in Transmogrification to hide slots.
However, choosing these items costs money to transmog them onto your gear, unlike the Hidden items, which are free.
They’re also, and that’s the issue here, selected by default if you select the “Hidden [Slot]” option. Since 9.0, you can at least right-click this entry to select the actual Hidden item, but that’s still not ideal.
The Hidden items, as well as (or especially) the currently useable Hat of the Xth Sister items (from Hallow’s End) show that specific items can be set to not cost money to use them in Transmogrification, please change the items mentioned above to be like that as well, or at least make the Hidden item the default selection (if one prefers the other items, they can be selected manually)