Strange Biomes Spawning

Hi there!

So, I'm having an issue with weird biome generation. I often add custom biomes to my mods (and the bug I'm about to describe happens in every project). When I create a biome that's supposed to spawn on the surface, everything seems to work fine at first. But when I create a new world to see how it's generating, and I use the /locate biome command, the biome often gets located at negative Y coordinates, like -59 or -62 (not always, but quite frequently).

And when I teleport there out of curiosity, it turns out the biome either isn't there at all or it's only a tiny sliver, just 2–3 blocks tall. I have no idea what's causing this. I haven't set it to generate in caves, and as for the generation settings, I'm using the ones provided in the documentation.

If anyone knows what's going on, I’d really appreciate any help or suggestions.