There was a change in the 1.17 pre releases that removes any hostility between mobs in peaceful difficulty.
Since this wasn't mentioned in the patch notes and as far as I understand it "peaceful" refers to mobs not attacking the player, I think this might be a bug.

I tested the following mob interactions:

• cats don't attack chickens or rabbits
• wolves don't attack llamas or foxes
• polar bears don't attack foxes
• snow golems don't attack shulkers
• shulkers don't fight back when attacked by a non player mob
• axolotls don't attack squid or fish
• iron golems don't attack shulkers or hoglins
• ocelots don't attack chickens

There are probably more that I don't think of.