Not aware of any changes in the spawning mechanics anytime recently, could you elaborate better about your first paragraph?

The chest spawning on top of the corrupt wood hut seems relatively normal.

Oh the chest spawned on a ship wood and slab hut on my killbox platform which is ship wood. It only happened once that I know of but the roof is about 6 blocks north and 4 blocks higher than the corruption platform.

The platform of corrupt wood is made on a RW8 world. There's a useful glitch of the wood at 1024 x 1987. I made the platform in the claim west of this glitch in the chunk 1 chunk west of the corner where the glitch wood exists. They are at y = 99. The kill platform is about 7 x 7, I think, and meets the claim edge of the glitch and is 2 blocks south of the north claim edge. I made a 1 block wide "bridge" from the killing chamber platform which ends about 4 blocks short of the corruption platform, so I can jump over to get chests easier. I made another platform of grass which meets at the corner where the teleporter is located to put them in the killbox. Both of the platforms are 3 or 4 blocks from the edge of the killing platform.

I've expanded the base since this spawning was going on...The world is called Join Please by Fireking. You'll know it's the right world because I have a small ship at my base which the corrupt mobs like to play on.