Silverfish spawning outside Mountain Biome

I was mining in the Plains biome when I happen to see two skeletons trying to shoot at me from a small hole in the cave that was one block wide. Of course I couldn't harm the skeletons with them being in a hole that was one block wide so subconsciously I broke the stone blocks so I can enter the hole and the stone blocks that I broke happen to be monster eggs I killed the silverfish  then checked which biome I was in and I was in the plains biome. I found that odd because silverfish do not spawn or atleast are not supposed to spawn outside of the mountain biome in Java Edition. After killing the silverfishes I dug straight up to see that I'm not in an extreme hill biome but am close to one. I did check to see if there was a stronghold near by and there is not one for hundreds of blocks. Is this a bug or are silverfish now supposed to spawn outside the Extreme Hills in Java.