Mobs that are usually aggressive to other mobs no longer do on peaceful mode
For example:
Polar bears no longer tries to attack foxes
Foxes no longer tries to attack chicken
Snow Golems no longer tries to attack shulkers
This bug was discovered by Ilmango on his pre-release 3 video: https://youtube.com/watch?v=QVHvd5IEQTA&t=4m14s
Linked issues
Comments

In other words, this ticket is a duplicate of MC-227058.

Dupe of MC-227058
See MC-227058