When connecting to a server, the biome borders (in the overworld) are not constant across all y-levels, unlike in a client/singleplayer world. This is easiest to observe when using the Biome Border Viewer mod, but can also be observed using the F3 screen.

Tested by comparing a client world and a server world with the same seed, the client world's biome borders were constant across all y-levels, while the biome borders when connecting to the server varied with the y-level.

https://github.com/mrp-v2/BiomeBorderViewer/issues/2 is the source of this issue.

Attached Images (taken using the referenced mod above):

world seed: 7031511839578461877

location of camera: -258.5, 73, 0.0 (-145, -17.5)

observe the smooth borders on the client world, compared with the rough borders on the server world.

Images taken without mod:

The ones with the default skin are taken on a dedicated server, the other ones were taken on a client world. Notice that on the server, moving up/down by a block changes the biome, unlike on the client world.