After testing step by step, I discovered that the reason units weren't responding to the addForce function was because they had already been killed (this was an issue with my mod). So the conclusion is: addForce doesn't work on units that have just been killed — it takes a few seconds before it starts working normally. During that time, even executing _man awake true has no effect. Sorry for wasting your time. I think this case can be closed.

Here are the conclusions from my further testing: