You are indeed correct, seems I shouldn't have trusted Spriter quite so much. I created the files with a3r1 but even a4r1 doesn't put ids onto non-tweened objects when it saves the file. (It loads and runs the files just fine though, so perhaps it is relying on something else.)
I'm away from the machine I used to create the files, but I'll update soon.
Thanks for the feedback,
Tim