Wow, I was just about to post this very question! I save a backup of a game, but now I'm testing this feature on an iPad and it will save it to a slot, but then I invoke a download (where I usually can select where to save it) and it won't do it.
I am not sure but give a shot with share plugin (I am not sure whether it will work in app export or not). Infact it is even better than what you are currently doing right now
Construct 3 will usually save the data in usual way. For the backup you can "share" the save file :- like in Google drive (or share somewhere else). It is much better than "download" as construct 3 is already doing it.
But unfortunately in my case it would be disastrous 😂. Why to use a app which can't even simply download an image?