I've seen this a number of times, too.   Not sure what steps/scenarios are causing it.  It only seems to happen when I'm not paying attention...
 

Has anyone submitted a problem report to Cakewalk? I would have but I don't know the exact steps it takes to replicate the bug. It usually happens to me if I double click on a MIDI clip opening the PRV, then if I drag the window as high as it can go (so the PRV window completely covers the track view), edit some, then return to track view--it usually disappears after that. I found that if I fool Sonar X2 and only drag the PRV window up about 3/4 so I can still see the now time marker and timeline it does not disappear. So for now I'm sticking to that!

I'm hoping for a patch soon that will fix this and other problems . With the holidays coming up, If we don't get one soon I wouldn't expect one till next year.