How do I see my sims Swimming?

I have a "similiar" things. My best shot is that Sims aren't being rendered if you aren't viewing the upper floors.

Sometimes I notice that when my sims go downstairs sometimes i can't even see them coming down until they reach the lower level.

Game will only render things on and below the current level, so I'm assuming that since the sims are technically on the "higher" level, they are simply not being rendered in order to optimize the game performance.