Animate Stacks stop previewing?

I can’t tell if I’ve done something to cause this behavior but all of a sudden - on a site I’ve been working on for a week or so - after a few minutes of working fine, any elements inside of a Foundation Animate Stack stop previewing. It’s not that the Animation isn’t working, everything inside the Animation Stack becomes invisible. Switching to new pages within the same project doesn’t fix the problem.

If I quit RW (v7.1.7) everything is fine again for a little while. Has anybody else run into this?

Mostly disconcerting because I don’t know whether or not it’s safe to save changes to a project when I quit since the preview is so far off from reality.