It can't keep exactly the same IDs because it is possible for it to potentially clash that way (happy to stand corrected).
I believe if you do what @roman_r_m suggests in your other thread then the relationships will be maintained.
I agree it makes more sense for it to default to single Jex (especially when it comes to new users) but that is for @JackGruber to decide, however even if not then there still could be a case for Joplin setting that option itself as part of the default plugins project.