I'm not able to reproduce this behavior. I only get this behavior if I visit node/add/page and then re-open node/add/page from the admin menu (effectively opening the page twice in a row). I think this is the expected behavior in this case, since the previous page was node/add/page
Backdrop does try to clear out unused files from certain folders periodically (it's called "garbage collection", no offense to your images).
You may wish to instead drag the images into...
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I'm not able to reproduce this behavior. I only get this behavior if I visit node/add/page and then re-open node/add/page from the admin menu (effectively opening the page twice in a row). I think this is the expected behavior in this case, since the previous page was node/add/page