Isn't that exactly what is happening now with a mirrored object, a change of coordinates when it's deformed?

No, if you open the door and close the modification dialog, the corrdinates will change. But then if you mirror the object, its coordinates won't change.

I doubt there are currently many existing homes with an opened mirrored object, and specifically one where the deformation goes outside the horizontal bounding box.

As this behavior was programmed this way from its first version, I guess that there are some users who experienced it and moved the door afterwards to make the door frame appear at the place they actually wanted it.
I also remember that updating coordinates of a deformed object was not easy to program, and fear that taking into account the mirror flag might be more complicate.
Nevertheless, I keep in mind this and will try to see if it can be improved without breaking existing arrangements.