in day, skeletons and zombies will randomly stop burning for a few seconds, even if there is no solid block above them. the max amount of seconds i've seen them not burn for is 5.

to reproduce:

1. spawn zombie or skeleton or both on a block they can stand on.
2. set time to day
3. make sure there is no solid block above them
4. you will see that every now and then the mobs will just stop burning