The bug
The /damage command sometimes returns a fail message when successfully damaging target. For example, damaging an armor stand.
How to reproduce:
For example:
summon an armor stand:
/summon armor_stand
damage it with out_of_world:
/damage @e[type=armor_stand,limit=1] 20 out_of_world
❌ The armor stand is killed, but returns a fail messgae
Can confirm this is still an issue in 1.20-rc1