This world contains biomes which are described as neutral but contain evil features; evil weather and undead wildlife.

It is no news that this can happen if you embark on an area that borders evil biomes, even if said biomes do not show on the pre-embark screen.
However, here, the closest evil areas are several embark tiles away. IIRC this never happened to me before.
This is the area to try:


Note that evil rain only gets announced: it doesn't fall anywhere. This probably means something.

I think I saw similar behaviour in other areas of this world, but I had little success replicating them.

Changes I made to the game: reduced frequency of certain minerals, removed aquifers from some stones/soils, removed certain pieces of clothing from entity default