I had 4  furnaces. 2 of them melting stacks of sand on top of each other(In the yellow box) and 2 furnaces next to them on the right melting cobblestone(In the gray boxes). After I had done some building and digging I went back to check them and in one of the furnaces (top left in the red box) I found a stack of cracked stone bricks where there should have been a stack of glass. In the other one below there was a stack of glass as there should be and the stone had melted normally as it should too. I attached 2 pictures I hope they help. The bricks can be seen in my hotbar with the stack of glass.This was very confusing.