The zombies in my mob farm are catching alight and taking damage, even though being out of direct sunlight and away from anything fire related. 

This problem is not constant, some do not catch fire, some do. I've tried rebuilding the top part to see if the light hasn't updated or something but to no avail.

I've included a link to a quick video I made to better show what's happening.

https://www.youtube.com/watch?v=Tp38rtJRMfU&feature=youtu.be