I have also experienced this.
Additionally, at higher bounty levels this mission is almost impossible to complete (completely impossible with a nearby Necramech). The Grineer are scaled so highly that all of them (specifically the Trooper grineer with the shotgun) are capable of one-shotting infested the exact frame they spawn in, mostly preventing completion but absolutely denying any chance of obtaining the mission bonus. I have tried several frames and weapons and nothing is really able to out-perform the reflexes and accuracy of a computer AI with knowledge of the exact frame to attack.
EDIT: I'd like to add that the consistency of being horribly outperformed by AI knowledge and reflexes of the Grineer is so routine, that I frequently just quit the bounty mid-stage and wait for it to cycle to a different set of missions, or just ignore the higher two tiers entirely until otherwise.