Minecraft – The case of the dead sheep

I have four compact wool farms, each of which features a sheep of a different color standing in a 1x1x2 area, completely enclosed with glass blocks, except for the bottom block which is obviously a grass/dirt block. The wool is collected by a hopper in a minecart underneath the grass block and moved into a chest. One day, I come back to the farm to find one of the sheep missing. I jumped around the glass blocks to make sure I hadn't forgotten to cover any sides which could potentially let the sheep escape. The 1x1x2 area where the sheep used to live was completely covered from all sides. Then I opened the chest. In it I found not only wool, but also two cooked mutton. This means my cheep has died of fire. So, what could have potentially killed my sheep?

I've had the following theories

• Some mob spawned inside the 1x1x2 area where the sheep was. However, I find this unlikely because the glass block above the sheep has a torch placed on it, so the light level inside the enclosure is 12 or 13. I don't imagine any nether creatures spawning in the overworld which are the only ones that could kill something with fire.

• Lightning? But then my sheep is enclosed in glass and under a roof. Glass protects from lightning, doesn't it? If it doesn't, and my sheep was in fact killed by a lightning bolt, how could one farm sheep safely? The reason I my roof is made of glass is that the wiki says that grass cannot grow under solid or light-blocking blocks and:

  Grass can grow under all other transparent blocks, like glass, fences, torches, or pistons.

1.14.4,Java edition, Survival mode, Normal difficulty.