I think this is the intended behavior. In marketing mat'l I've seen for iOS 8, they make a point of saying the editing you do on the device does not affect the original.
I know in iOS7, the edits applied to a photo were maintained in some instances of transfer off the device, and not in others. For instance, if I edited a photo, then texted it, the edited version would transfer. But, if I connected the iPhone to my computer, and transferred contents of the camera roll via drag and drop, the original would transfer.