"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.
1. Summon a raid leader:
2. Give it some armor
3. 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.