I notice that if your strength bar is lower than the AI driver's that you switch to, the bar will slowly rise to the amount of strength they have, making it seem as if the bar is going up. But in actuality, it's just moving up to meet the exact amount of strength that they have.
That's the only time I see the str bar going up, unless you're talking about it going up a few minutes after switching to the AI driver cam, then idk...