There is an issue with the EQUI_ItemDisplay.xml file when attempting to use Hero's Forge ornaments in the type 20 slot.
It appears that the visible slot and the area to drop the ornament onto to use it are not always aligned correctly.
Have seen this where one item has a 'Race' line that is only a single line (the drop area starts approx 1/3 of the way down the box) vs an item with a 2 line 'Race' where the drop area is aligned with the visible box.
See, Guardians Mace to see the problem vs Heroic Spiked Knuckles for correctly aligned area.
I do not think this can be corrected by a custom UI because so much of this window appears to be hardcoded.