The bug

The old sniper-duel achievement was triggered when killing a skeleton at a minimum distance of 50 blocks. The new sniper-duel advancement instead can be triggered by killing a skeleton at close range and subsequently killing any mob, skeleton or not, at a minimum distance of 50.

Steps to reproduce

1. Revoke the "Sniper Duel" advancement in case you already have it

   /advancement revoke @p minecraft:adventure/sniper_duel
   

2. Give yourself a bow and arrows
3. Shoot & kill any mob (except skeleton) at a minimum distance of 50 blocks

   /summon husk ~ ~ ~50 {NoAI:1b,Health:0.1f}
   

   → You are given the "Sniper Duel" advancement

Possible fix

Include "type": "skeleton" in the "entity" section of sniper_duel.json.