App exposé sometimes makes all windows disappear!

Has anybody seen an issue whereby using app exposé, via a four-finger down swipe gesture, sometimes causes open windows (eg Pages documents) to properly display, and sometimes causes all the windows to disappear? I can see a pattern to it, and I can't think of what the disappearing behaviour might be useful for, so I guess its a bug. I see it with Pages, where it happens 5/10 times, but I could replicate it with Safari, so maybe it is just a Pages issue.