Digiscope Warning - Canon A710IS

May I pass on the following cautionary tale to any digiscopers out there?
As some of you may know, I have always used the hand-held technique for my (limited) amount of digiscoping, and late last year I decided to replace my Canon A85. After much research I settled on the new Canon A710IS, which has a 6x optical zoom and an image stabilisation system, and purchased one from Jessops in Manchester.
Vignetting was not a huge problem with my old set up, and zooming in a bit soon got rid of any dark corners. I got the new one home and tried it through the scope - wow! A lovely, full picture at normal magnification. BUT, zoom in, and it rapidly disappears to a small disk (a bit like zooming out fully on Google Earth, for those who've tried it!), and only returns to anything like a full picture on the maximum zoom.
Due to work (and lousy weather) I have not had enough chance to fully assess the situation, and maybe the increase in pixels will enable me to use the normal magnification setting.
I presume other higher magnification compacts may exhibit the same problem, and of course, it never shows up in the reviews you read! Incidentally, the camera is excellent in every other respect.