After we upgraded our Bedrock server to the Nether update, we encountered changes to the world generation for chunks we'd previously discovered. Instead of seeing the expected biomes, we saw mostly stone. In many areas, we saw clear borders where we saw what we expected on one side, and mostly stone on the other.

See attachments for screenshots of the weird world gen. We were fortunate enough to be able to dig up a few "before" screenshots to compare with the "after" screenshots. We also have a few screenshots of underwater structures that are now sitting on stone, a weird chunk border, and an example of a map area we explored that used to be more diverse, but is now mostly stone.

This is on seed -46777102.