Alright, so this happened when I updated BOP from 1.15.2 to 1.16.1.

For some reason, the borders between generated biomes are weird. Like, they're perfectly straight and form "squares". I'm beginning to suspect something messed up when I updated BOP, because I haven't found a BOP biome in a while. However, this issue is contained only on my primary survival world, as I can still create BOP configured worlds.
It's not a huge deal, but a fix would be appreciated.

Existing worlds from 1.15 are not going to work with new chunks