I am not sure that this is an issue but it bugs me and this is the first time this has happened for me. Not my first spawner xp farm but first time glass was blocking the flames.

I was creating an xp farm from a pair of two spawners I found and started to place glass on the front of them. Went to check that my placement of where I plan to stand and noticed that the glass totally blocked the flames that indicate that the spawners are active. Got closer and noticed that the flames were only blocked by glass.