Mobs can hide in double plants to prevent being burned. Skeletons automatically finds its path to double plants though zombies seems like they want to find a better spot (Underneath a block).
MC-123853
Mobs don't burn when standing in double plants/cobweb
Resolved
Fixed
11
Maartin
Confirmed
burning, double-plant, sunlight
Minecraft 18w01a, Minecraft 18w02a, Minecraft 18w03b, Minecraft 18w05a, Minecraft 18w07c, ..., Minecraft 18w10c, Minecraft 18w10d, Minecraft 18w11a, Minecraft 18w14a, Minecraft 18w14b
Minecraft 18w15a
Retrieved
Note: The sunlight height map is cached so you may not notice the bug fix in existing worlds