Stained glass appearing through potion effects

Tested with lingering potions.

After throwing down a bunch of lingering potions (To create a "cloud"), I looked at some stained glass in my world. To my surprise, I was able to see the stained glass! nothing behind it, just the glass. I tried it with regular glass, and it did not work.

(P.S. The 2 blue rectangles are stained glass.)