Mine stayed in the first time until he got the moodlet required, but he got up the second time (to my surprise). I had and still have autonomy off, so I can't tell what makes a Sim stay put or not. It might be a bug? There were other Sims in both times. But I've noticed that autonomy doesn't always work in some situations in this blasted game.
Update:
It would seem your Sim can get the necessary moodlet for the challenge even though getting up. Just keep forcing them to get it.