mobs disappear when behind glass and 5-10 blocks away but reappear when you move closer

I was playing around with the lightning rod and went back a few blocks and the creepers I was testing with disappeared. Upon further examination, I discovered that they would become invisible when you are 5-10 or so blocks away and behind glass. The attached image shows an invisible creeper behind glass. You can see the top of its head at the top of the tube. Thanks.