Im actually working on a datapack for 1.17, so if this is fixed by 1.18 please let me know, but apparently Im not the only one that has experienced this.

For some reason, I have run into chickens a few times in the middle of deep_lukewarm_ocean biomes. I thought at first maybe it was part of a baby zombie chicken jockey, but there was multiple chickens and I have seen this happen multiple times.

Edit, it cant have been a chicken jockey because as I mentioned, i am working on a datapack and this was immediately after world generation.