I dont know how it works it out tbh but i agree it should save/load like for like. I know there is some new things with ui widgets that get items in slots ect but i dnt think its past that nothing to my knowledge lets you get/set slots.
After you explained i sorta get the reasoning now and it makes total sense why you want this. If i were you i would create a post on the requests forums as this will not be seen as a bug i think but either way ill vote as this would be better for sure and tbh until i read your post i thought it save/loaded in the same slots by default