Can't help you with understanding it, but I've observed the same thing.
Try this: float a PRV window. Press B (the browser will, maybe appear or disappear). Press B again (maybe it works) Press B again: nothing happens. Now minimize the PRV window, and then bring it back to focus. Press B: it works again, once or twice, then stops working.
I always thought it was because I had a B shortcut for "bounce to tracks' in Track view, but maybe there's a more general problem.
post edited by williamcopper - 2016/09/29 07:59:35