Heads & skulls aren't rendered when seen through stained glass in spectator mode on fast or fancy graphics

When looking at a mob head through stained glass in spectator mode it fails to render properly

What I expected to happen was...:
heads should appear

What actually happened was...:
heads were invisible

Steps to Reproduce:

1. Switch to "Fancy" graphics. This problem does not appear with "Fabulous!" graphics.

2. Put a wither skeleton skull on any block. I used Cracked Deepslate Tiles as the block.
3. Place gray glass panes horizontally around the wither skeleton skull.
4. Change the game mode to spectator mode. If you look through the gray glass pane the wither skeleton skull should be invisible.