Mobs burn under blocks by the sun.

It turns out I have a skeleton XP Farm located in a long distance away from my base, so I use nether portals to travel around the world. Since the Mob Spawner is kind of in the surface of the world I had to take the mobs out of the underground and make them fall 21 blocks so the farm could work.

At first, it worked well, lots of XP and a considerable amount of loot. But when I went to my base and returned through the nether portal the skeletons were burning.

I may have used a flint and steel to burn them one time. But I have analyzed the behaviors of the mobs and got to see one golden helmet break, this one belonging to one skeleton with armor, and therefore the mob started to burn.

To fix this, I tried to cover the ceiling with 3 layers of blocks, but it still acts as a transparent block letting the sun go through.

Every time I go to the farm I have to go up there and take out the blocks above where the skeletons are standing and put them back again.

Is this a bug? are there new properties of mobs I don't know of?