There are numerous Bug reports on Linux about Tablet bugs, i.e cursor jumps. To me it seems, that the underlying cause of most of these screw ups is a change of the coordinate scaling in the driver interface, which has not been properly changed in the Qt interface. It seems that by not regarding the new scale factor in Qt the pen is now mapped to a tiny area of the screen in the upper left corner, then Qt gets further screwed up, reverts to mouse mode and the coordinate buffer overflows.

See full details here, as the bugs appear to me:

http://sourceforge.net/apps/mantisbt/ipe7/view.php?id=62&nbn=6#bugnotes
( Ipe bug report)

http://old.nabble.com/Ipe---Qt-Pen-interface---canvas-coordinate-jumps-to32435132.html
( Linux driver developers forum )

I am not familiar with Qt, but did the reporting. I think the bug is pretty well nailed down.
Further questions should be directed to the Diver developers.