We have a custom behavior pack that gives Rabbits a melee attack on the player.  This was working in 1.7.1, but does not cause damage to the player in 1.8.0.24.

nearest_attackable_target is working as expected.  The rabbits detect and chase player.

attack is set to damage: 1.

Rabbits will swarm player, but will not deliver damage. (Note:  Rabbits are custom-modeled and only have cubes on the "body" bone)

Interesting observation.  Although no damage is delivered, if I add the "effect_name" and "effect_duration" to the attack component, it will deliver that status ailment to the player.