Official comment
The back button works, it is just a bit small. We are working on a bigger interface. We optimized the layout to work with phones first. It is always easier to make it bigger than it is to make it smaller.
EDIT: http://support.dronesmadeeasy.com/hc/en-us/articles/205918496-Bug-Workaround-Back-Button-on-iPad-
Aaaahh. Now we see what is going on. We have just learned that on some iPads, the status bar is showing and is covering the Back button. This was not an issue in our iOS 8 development and none of our 400 beta testers reported it on iOS 9... The bottom part of the circle works so don't focus your pushing in the middle of the admittedly already small button. Try to push the very bottom of it. This will be fixed in our next release.