When using the /damage command to make one zombie hurt another, the attacker zombie will seemingly fight a ghost, get hurt by it and die.
The zombie that was hurt will try to walk up and attack the attacker zombie as well like normal.

Expermiment:

Zombies with tags are spawned at two sides of a barrier box:
Spawn Attacker: /summon minecraft:zombie 93.50 -60.00 73.50 {Tags:["attacker"]}
Spawn Hurt: /summon minecraft:zombie 93.47 -60.00 62.49 {Tags:["hurt"]}

Damage command to make them attack each other:
damage @e[tag=hurt,limit=1] 0 minecraft:mob_attack by @e[tag=attacker,limit=1]

Result:
When the attack zombie hurts the "hurt" zombie, the attack zombie seems to start fighting a ghost and being attacked by it. This results in the attackers death in 100% of cases.

The same effect happens even if there is a barrier or a wall inbetween both zombies, making interaction imposible.

Was also tested on Creeper and Skeleton but these mobs did not show this effect, even when combined with a zombie