It can be reproduced:
1. Open a new world
2. Use /locate biome savanna_mutated
3. Go to the coordinates.
4. Do /weather rain
This is on Windows too.
This still works. It also works with fences.
1 zombie pigman and 2 drowned were ejected from boats in my world.
It may be intentional, but it can be replicated.
I have sucessfully replicated this bug.
You have to dig down from the top until you reach more suspiscious gravel. This is not like in Java Edition, so it is probably a bug.
Please read the description.