Viewport panning strange in crop-box

[SSKI]

Hi,

I'm having problems with a buggy viewport in Revit 2015.

Below is a animated screenshot of a normal viewport on a sheet when panning the sheet view towards left on my computer.

http://imgur.com/a/dN7vQ

Below is the buggy viewport on a sheet, panning the sheet view in the same way. See how the crop box moves normally but the view inside it doesn't move along with the crop properly.

http://imgur.com/a/o6ruL

What is causing this problem? I could just make the same view again but I would like to know what is causing it, I've had this happen before.

Thanks in advance,

S

EDIT: this is a section view, copied from another section.