The bug
When the entity inflicting the damaged in the /damage command is a some mobs, the entity target to be damaged will not receive additional status effects.
Reproduce steps and affected mobs
Wither skeleton
/damage @p 1 minecraft:generic by @e[type=minecraft:wither_skeleton,limit=1] from @e[type=minecraft:wither_skeleton,limit=1]
Wither skeleton with bow
/damage @p 1 minecraft:arrow by @e[type=minecraft:arrow,limit=1] from @e[type=minecraft:wither_skeleton,limit=1]
Husk
/damage @p 1 minecraft:generic by @e[type=minecraft:husk,limit=1] from @e[type=minecraft:husk,limit=1]
Stray
/damage @p 1 minecraft:arrow by @e[type=minecraft:arrow,limit=1] from @e[type=minecraft:stray,limit=1]
Cave spider
/damage @p 1 minecraft:generic by @e[type=minecraft:cave_spider,limit=1] from @e[type=minecraft:cave_spider,limit=1]
Bee
/damage @p 1 minecraft:generic by @e[type=minecraft:bee,limit=1] from @e[type=minecraft:bee,limit=1]
Pufferfish
/damage @p 1 minecraft:generic by @e[type=minecraft:pufferfish,limit=1] from @e[type=minecraft:pufferfish,limit=1]
Burning zombies, zombie villagers and drowneds
/damage @p 1 minecraft:generic by @e[type=minecraft:zombie,limit=1] from @e[type=minecraft:zombie,limit=1]
Expected to happen...
When the entity inflicting the damage is a wither skeleton, the target gets the wither effect.
When the entity inflicting the damage is a wither skeleton with bow, will cause the target to catch fire.
When the entity inflicting the damage is a husk, the target gets the hunger effect.
When the entity inflicting the damage is a stray, the target gets the slowness effect.
When the entity inflicting the damage is a cave spider, bee or pufferfish, the target gets the poison effect.
When the entity inflicting the damage is a burning zombies, zombie villagers and drowneds, will cause the target to catch fire.
Confirmed. I've altered the title to fix grammar errors as well as to make the problem a bit more clear. I hope this is okay. 🙂