Description:

When you ride an entity with the /ride command, using the /damage command while you ride said entity makes your vehicle target that specific entity, but it does not actually inflict damage to this target.

How to reproduce:

1) Summon a spider: execute summon spider run tag @s add spider

2) Summon a skeleton: execute summon skeleton run tag @s add skeleton

3) Ride the spider with the /ride command: ride @s mount @e[tag=spider,limit=1]

4) Attack the spider through the skeleton with the /damage command: /damage @e[tag=spider,limit=1] 1 minecraft:generic by @e[tag=skeleton,limit=1]

5) Watch as the spider goes after the skeleton, but fails to do any damage

6) If you dismount, watch as the spider can now successfully attack the skeleton

What is happening:

The spider is chasing the skeleton but fails to do any damage. 

What should be happening:

The spider should be inflicting damage onto the skeleton.