Steps to reproduce:

• Let a Zombie drown to get a Drowned Chicken Rider
• Try to hit it - no effect
• Stay in front of it - it won't attack

Some remarks:

• Screenshot attached
• The Drowner wasn't spawned by Egg or by Command, it was a "regular drowned Zombie"
• The Drowner was at night converted from a Zombie by pushing the Zombie ito the sea
• Meanwhile it was daytime and raining
• The rider didn't do anything special except having the usual chicken idle movement.