I was playing on a superflat world in the first 1.20.3 pre-release and I tried to use the fillbiome command /fillbiome ~5 299 ~5 ~-5 319 ~-5 

minecraft:snowy_plains and it came back with this:

It's not just a text error, the game actually changes the biome 

This bug also occurs for

/fillbiome ~5 297 ~5 ~-5 319 ~-5

/fillbiome ~5 298 ~5 ~-5 319 ~-5

and for any vanilla biome (haven't checked using datapacks or other custom biomes)

Can also confirm 1.20.2 and 1.20.3 pre-release 2

Does anyone have any idea why the game outputs that the first y value was 296 even though I made the first y value 299?