The Bug

Maces are not seen as a better weapon by mobs, which is seen if they are already holding a weapon that does less attack damage than the mace. They will also downgrade their mace for a worse weapon.

How to Reproduce

Dig a hole that is two blocks deep, and then summon a zombie using the command below:

/summon minecraft:zombie ~ ~ ~ {CanPickUpLoot:1b,Invulnerable:1b}

Throw the zombie a wooden sword. Then throw it a mace.

Note how the zombie does not substitute its wooden sword for a mace, despite the mace doing more damage.

Kill the existing zombie and summon a new one using the command above. Throw it a mace. Then throw the zombie a wooden sword.

Note how the zombie prefers the wooden sword, and drops the mace for it.

Additional Notes

Related to MC-193176.