Skeletons catching on fire when under shaded area

Currently I'm playing on a server in the latest version of minecraft and I made a xp farm out of the skeleton spawner. When the skeletons would land at 1 hp they will catch on fire for no apparent reason even though I'm under ground. Before the bug occur the farm was working without any problems. 

What I expected to happen was...:
This is self explanatory. 

What actually happened was...:
What I stated in the summary. 

Steps to Reproduce:
I don't know it could be the server