Hi,
when using Mini Viewport, it is cropped from the bottom quite a bit, so one can't gauge, what is the camera really filming. 
When I make mini viewport smaller it is cropped even more, so It looks related. But the Mini Viewport is impossible to make larger .. it can be only enlarged more even when as separate window.

Put simply - main viewport is different than mini viewport. 

Any ideas? I guess this is most probably a bug.

Thanks,
Jirka 

Just checked mine and it works fine.  Changed the resolution, switched cameras, all good. 
Are you sure you have the correct camera selected in the mini viewport?  Perhaps post a screen so we can see the issue.

93% of original size (was 539x16) - Click to enlarge93% of original size (was 539x16) - Click to enlarge