Steps to Reproduce:
1) Run /damage @s 20 minecraft:generic by @s
Note that the death message appears as "Player died"
This should be "Player died because of Player"

2) Shoot yourself with a bow and arrow then run  /damage @s 20 minecraft:generic
Note that the death message appears as "Player died because of Player"
This is the correct and expected death message

Note that adding "by @s" in the 2nd step gives "Player died" which is wrong.

You can replace "minecraft:generic" with the following for the same results:
minecraft:cactus
minecraft:cramming
minecraft:dragon_breath
minecraft:drown
minecraft:dry_out
minecraft:explosion
minecraft:fall
minecraft:falling_anvil
minecraft:falling_block
minecraft:falling_stalactite
minecraft:fireworks
...