The Proximity III assignment (and presumably any of the similar artifact scanner "combat" assignments) can very easily become impossible to complete, as follows.
If the artifact is located near the edge of the marked area then we are extremely likely to step out of the circle when killing one (or both) of the spawned enemies. Such kills are not credited to the assignment objective. Since there are only just enough spawns to complete the objective at all, and no respawns for this assignment, there is simply nothing the player can do but abort.
Personally I don't understand why you made it so that we have to kill the enemies while standing in the red circle for ANY assignment. Doing so often forces you to abandon the very tactics that the game is trying to promote.