I confirm this issue.

We run;
- Server side;
- PyQt4-4.11.3-gpl-Py2.7-Qt4.8.6-x32
- Windows Server 2012 R2 Standaard
- Client side;
- Windows 7 Professional
- image.png

I also note that when I drag a small enough (so preventing the issue) button in the RDP session it is invisible. When I drop the button it becomes visible.
When I do this client side the button remains visible while dragging.