The bug

If you kill a pillager using a crossbow with a firework, you don't get the "Who's the pillager now?" advancement. But you do get bad omen and "Voluntary Exile" advancement after killing a raid captain.

How to reproduce

1. Take a crossbow and a firework in the offhand
2. Load the crossbow with the firework, shoot at the pillager using fireworks until he dies
3. Repeat same with raid captain

Result: You don't get the "Who's the pillager now?" advancement, but if you kill a captain, you do get the "Voluntary Exile" advancement and the bad omen effect but not the "Who's the pillager now?" advancement.

Checked in creative and survival, enchanted crossbow and normal crossbow.