SketchBook can't flip an image 90%. When you flip the tablet, the UI will flip along with it. It doesn't flip the UI and canvas.
To fix issue, you can copy the file to the desktop version of SketchBook and rotate it.
The folder is here: /Autodesk/SketchBook3 > Connect via USB
Download the FREE version here to edit the file: www.sketchbook.com/thankyou
David Lau