I don't know if it's a bug, but the snipers that used to kill with one shot now don't kill anymore.
Are you talking about L96A1? It never was guaranteed "one-shot".
Damage depends from many factors.
1) Are you playing Hardcore, or Normal mode.
2) Distance (at some distance damage will drop)
3) Where exactly bullet hits (head, torso, arms, legs)
4) Enemy health (enemies can have more then 100 HP)
5) If you use suppressor. With suppressor damage drops faster.
Do you own tests on Practice/Sandbox map.