In a structure built in a swamp biome that is lit end to end with excessive torches I have had groups of up to 3 creepers spawn simultaneously, this has also happened with single spiders and in a different world groups of up to 3 zombies. Similarly oddly in the same area I have had creepers explode outside of the structure and break all the signs, boxes, stairs and chests inside the structure but none of the structure's Granite and diorite blocks (Which is the primary building material of the structure) blocks. interestingly the creepers behave as if they can see me through the structure's walls and explode despite not being in range of me to normally do so. The floors in the room they are spawning in are smooth stone. This has only started happening in worlds created since the most recent update.