When you use a plug-in or an external editor, you break the lossless workflow. Aperture will create a new original file(a psd or tiff file) from the version you are editing. And this new original will be sent to the plug-in, and this original will be the one you are seeing when you toggle with "M". So after a roud-trip to the external editor, you can anly toggle back to the new quasimaster(original) with "M", not all the way back to the imported original.
You cannat avoid this, only by doing all edits with Aperture provided adjustment tools and avoiding external edits.
I would recommend turning off Stacks and testing -- that way you can see exactly what is happening. The "real' original is still there (unless you deleted it), as well as the original that was created for sending to the plugin or external editor. You can toggle to the original, but only from a version other than the one round-tripped.