Zombies unaffected in desert while its raining during the day

I pillared up in a desert to go AFK for a bit while my cattle bred below. At some point a thunderstorm started up and it began raining (I could see it raining in the plains next to me). Day broke and I came down from the pillar. Here's the bug: all the mobs that had spawned overnight in the desert did not burn up in the sun even though it wasn't raining in the desert.

Screenshot is attached. You can see how a horde of zombies are following me and not burning up even though its daylight and there is no rain. I created a new creative world and tried to reproduce this but I couldn't - so not sure exactly how the bug starts.