If I edit a file (without saving it…) and then decide that I want to reopen the previously saved file again I get the edited file.
No, this isn’t a bug: if the file is already open, then Dorico simply switches to the window that contains the file in question. If you want to re-open it without your saved changes, close it without saving it, then re-open it again.
Or save it under a new name and then open the original file?