black out glass

This is a bug that I found while creating an interesting item-frame based design tool.

1.When you place a item frame on a chest and then another behind it [see below],
2. place a block in the back one and a stained glass pane in the front,
3. the block in the back disappears.
P.S. only with stained glass not basic glass pane.
This is a small visual bug and nothing more