Very hard to work out from your given info. Do you get the save compatible PDF thumbnail dialogue as shown here? What happens when you open a new doc. then "save as" through file menu, give it a different name. Then when you open up your original doc., select all (Ctrl+A), then copy (Ctrl+C) from your actual drawing, paste it into the new doc.(Ctrl+V) and hit Ctrl+S does that save your drawing when you reopen it.
Work Flow: you should always save and give your drawing a name just after opening you new doc. There're a few other things that you could try but do this first. Also I don't really understand your bitmap attachment is this part of your drawing, is it tables that you are doing.
When you use the Selection Tool and select it through using the marquee does the extra object show up then (Blue).
Design is thinking made visual.
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