I am unsure if this is a true bug or an intended mechanism in the game, but I come across a weird circumstance where some hostile mobs spawn out of dispensers while in peaceful mode to their corresponding mob eggs. Usually while the player is on 'Peaceful Mode' if the player or entity activates a dispenser with a hostile mob egg in it, it will not spawn in the world. However I come to find that some mobs do spawn such as the Evoker and the newer mobs such as the Ravenger and the Piglins can while the world is set in Peaceful mode. 

This can be difficult for those who want to create an adventure map but need to watch out not to set off the dispensers so the mobs do not accidently spawn in, which will require the player to correct the incident and replace the mob eggs back to the dispensers. I have went back several times to see if this is still reoccurring and indeed it has not been addressed/noted.