Plains was not flat

This is expected behaviour: as of 1.18, biomes do not affect terrain generation (i.e. the shape of the landscape), which includes how flat or hilly the terrain is. Instead, these values are now controlled using multi-noise parameters. From the 21w37a blog post:

The terrain shape and elevation varies dramatically, independently from biomes. For example, forests and deserts could form up on a hill without needing a special biome just for that

For more information on 1.18 worldgen, check Henrik Kniberg's YouTube video.