Light gray stained glass panes do not appear in inventory slot

When i grabbed Light gray stained glass pane. i noticed when i stuck it any one of the lower inventory slots, it did not show up.

What i expected to happen:
The item icon wold show up in the inventory slot

What really happened:
It was transparent

How to reproduce:
Open inventory
Go to decorations tab
Find light gray stained glass panes
put in inventory hot bar (the nine you hold in hand)

Notice it shows up in game, but not in inventory