Happened again, this time on Exodus. Traitor mission complete and no rewards. The mission disappeared from the mission toolbar and no new Vynora mission appeared and no "attention of" message appeared.
This time I was right next to it and killed it with melee weapon.
[10:10:21] You target the mature Bull (traitor).
[10:10:23] You feel the presence of Libila.
[10:10:32] You stop attacking.
[10:10:32] The mature Bull (traitor) is dead. R.I.P.
[10:10:21] You try to cut mature Bull (traitor).
[10:10:27] You cut mature Bull (traitor) deadly hard in the left hoof and damage it.
[10:10:28] The attack to the right underarm glances off your armour.
[10:10:32] You cut mature Bull (traitor) deadly hard in the chest and damage it.