First this said this was fixed but it is still there in 1.15.2.

So look at the attachments part in the photos as you see that the light gray stained glass in my creative inventory is not visible but the yellow is. when not in your inventory then you can see it.

How to make the bug happen, just get a stained light gray glass pane in your creative inventory

and then you can see it.

That is the stained light gray glass pane bug.