The bug

When the source of damage in the /damage command is not the player, the damage will not be applied to the ender dragon.

Reproduce steps

1. Summon a non-player mob(e.g., enderman).
2. Summon an ender dragon.
3. Use the following command:

   /damage @e[type=minecraft:ender_dragon,limit=1] 500 minecraft:generic by @e[type=minecraft:enderman,limit=1] from @e[type=minecraft:enderman,limit=1]

4. Found that the system prompts the damage has been applied to the ender dragon but not so, the ender dragon looks unharmed.
5. Use the following command:

   /damage @e[type=minecraft:ender_dragon,limit=1] 500 minecraft:generic by @p from @p

6. The command works normally when the source of damage is found to be the player.