The bug

Hostile monsters that normally would catch on fire in 'clear' sunny weather, still are attempting to catch on fire and burn in 'rain/stormy' weather. This starts with vanilla server 1.8.0 and happens with all versions after that so far I've tested.
It's easily noticeable by the constant 'fire/water sizzle effect sound' that you get when you put something on fire out with water. (like mobs swimming in sunlight) Thought maybe it was a client sync issue at first so re-logged also to verify it still happens. This does not happen in server/client 1.7.10 or older.

You can hear the fizz sound and it uses durability of the item the monster is wearing in its head slot

/summon zombie ~ ~ ~ {ArmorItems:[{},{},{},{id:"diamond_helmet",Count:1b,Damage:363s}]}

Code analysis

Code analysis by marcono1234 can be found in this comment.