This can sound a bit silly but I made a spawner before and also yesterday which can spawn in every game tick if there is a player in a 1 block range. I used this to spawn a lot of mobs but only if I want. If I switch to peaceful, and step on the spawner, it will keep trying to spawn a hostile mob (in my case it's actually a FallingSand riding a creeper) and if I stand there for enough time (5-10 seconds) it will leave a creeper model there (I didn't check with other monsters but I think it will be the same), but it is not actually in the game. For example I tried to hit it, nothing. I tried to blow it up, the blocks blown up, it fell down to have a support block, but nothing. It doesn't move.

If you want to see what I did and what is the bug, there is a video I made:
https://www.youtube.com/watch?v=FI_FSd_y2Rc

I hope that this isn't a problem, I only posted my video to show the bug.