In the snapshot 21w16a, glowing squids and axolotls spawn in the bottom of my creeper farm, and I don't think they're supposed to. There was also a glow squid in a river in a swamp biome. I don't know how the spawning works for those mobs, but I thought they generally spawned in, like, deep oceans and underground water sources and such. I read the fixed bugs for 21w17a, and it didn't mention it, so I'm not sure if it's a bug or not, but I did want to mention it.