I have flat world where I do all my redstone testing and snapshot testing and just today I realized that on peaceful, slimes will appear and disappear right in from of my eyes. I tried restarting my minecraft and it was still happening. (it doesn't happen so very frequently but it happens enough to start distracting me and annoying me). It's as if the game is spawning them by default and then realizes that it's on peaceful so it deletes them like when you try to spawn a hostile mob whilst on peaceful mode. (it appears then disappears immediately)