glass (stained glass mostly) and a few other blocks turn into their own light source randomly

Put the summary of the bug you're having here
stained glass emits light somtimes like glowstone
What I expected to happen was...:
Describe what you thought should happen here
the room would be completly dark
What actually happened was...:
Describe what happened here
the room was fully lit up with light coming from the stained glass
Steps to Reproduce:
1 ...just make a dark room with stained glass ans a redstone lamp behind it but no light sources except for redstone lamp
2 ...make the lamp flicker on and off and for a few seconds and eventually the glass will just light up like glowstone even though the lamp is off