I only noticed this behavior when the enemy was being healed or damage boost by Mercy. A full charged headshot on an enemy widow would leave them at 50 hp.
Do you ever hear the armour dink by any chance? Sometimes I hear it when shooting non-armour enemies, it makes me think maybe the game is not calculating damage correctly in some odd scenarios
Falloff range.
Widow does not have unlimited range anymore.
250HP can be oneshoted within 56m
200HP is around 66m
and for 300HP (without armor) you need to be within 50m,
Heros like bastion or Torbjörn cannot be oneshoted, since they have armor