First of all, I am unsure of whether or not this is a real bug or related to something I did. I’m also not entirely sure of the version, but it is the latest one (The Garden Awakens), and I never noticed anything similar before that update. It has also never happened with passive or neutral mobs, and it has only happened in structures. I first noticed it in my hardcore world (where I only play with multiplayer off), when I went to a woodland mansion. I ran around, killing all the vindicators and evokers, but I started to hear a bunch of noises coming from behind the wall to the room with the staircase to the third floor. I opened up the room, and there were nine vindicators and two evokers hanging out in there, and they all started to attack me at once. Later in the world, when I went to a stronghold, a similar thing happened. I lit up most of the entire surrounding area, but I’m pretty sure there were also caves around the stronghold. I found a small dead-end hallway, where three creepers, two zombies, a skeleton, and a spider were waiting to attack me. Much less scary, but still a surprise. There were almost no monsters anywhere else nearby, even before I placed torches. Unfortunately, I don’t have any images or anything of either instance, but I have also only ever seen it twice and only in my hardcore world.