(Probably in older versions too, but I've seen it in version 1.19.2, Java). When creating single biome worlds, biomes are mixed. Specifically I made 4 worlds: one is just deep ocean, one is just deep cold ocean, one is deep warm ocean, and one is deep frozen ocean.
All the worlds are created with the respective biome as it should be, but the problem is that the biome always appears mixed with what seems to be a lot of fields and mountains; MANY landmasses, which doesn't make sense.
Something similar will surely happen on other single biome worlds. I think a correction on that would be timely.