Zombies falling out of a waterstream start burning in the daytime, even though they have a roof over their heads

What I expected to happen was that the zombies should fall back down in the water and start climbing again, without taking damage

Instead they start burning on the way down

Steps to Reproduce:
1. Create a zombie farm with a spawner. In my case tere's a zombie spawner ans a spider spawner
2. Create a wide bubble column with soulsand. Block the secons-to-top watersources with open fencegates in the stream direction, so water can flow over them, leaving enough space for the spiders. Put a roof over the whole structure.
3. Not all zombies will make it over the fence gates, the ones that fall start burning.