One of my favorite features of Fresco was that I could drag images around the art board to use as references, rather than have all images restricted to the area of the canvas. This was extremely useful and convenient and one of the major reasons I pay for Fresco rather than use a competitor's software for iPad. I can't seem to do that anymore--images disappear when I drag them off the canvas. Was that feature removed?
It is still possible to view content off canvas in the Windows version so it might have been something that was unintentionally changed on the iPad. I can definitely see how it would be a useful features to have the reference materials in app on an iPad due to screen size. I would suggest contacting the developers directly through the app via the 'Report a bug' feature which opens your email if you feel it is a bug. Or through the Suggest a feature option which opens the Fresco Uservoice website where you can ask for the feature and hope that enough others upvote your request.
Hopefully, since it was possible before, the developers agree that it is a bug and work towards reimplementing the possibility to view images outside the canvas on the iPad.