The bug
"fatal" damage is a filter used in entity behaviors. It's supposed to detect when the damage dealt is enough to kill the relevant mob. Apparently, this check is done before certain damage reductions, including armor. As a result, it's possible for this filter to pass even if the damage does not kill the mob.
How to reproduce
1. Summon a raid leader:
/summon vindicator ~ ~ ~ minecraft:spawn_as_illager_captain2. Give it some armor
/replaceitem entity @e[type=vindicator] slot.armor.chest 0 diamond_chestplate3. Punch it exactly 23 times
→ ❌ You got Bad Omen without killing the mob. If you clear the effect and punch it again, you'll get the effect a second time.
Linked issues
is duplicated by 4
relates to 3
MCPE-99884
Healing mobs right as they die causes them to come back weirdly
Resolved
MCPE-108988
Entity component: damage_sensor with has_damage/fatal will fire events immediately on damage from blocks
Resolved
MCPE-167651
No knockback or damage immunity when dealing massive damage against someone with a small damage multiplier
Resolved
Can confirm it's present in the current stable build, 1.16.20.3. Please get this issue resolved!