Hi Max,
We have this feature that you can use the artboard around the main frame (box with shadow borders in the middle) for keeping your objects. It is often quite useful to store some temporary elements there that you do not wish to include into the finished design.
Therefore, everything which is not inside the main frame is not included when you share or export the project.
Sometimes, though, it happens, that the objects are located above the main frame, but not inside it. For example, you can accidentally drop the element outside the frame and it will lie on top of it, but not inside it. Perhaps this was your case?
(You can use the Outliner panel in the bottom left corner to see where your objects are).
We are working on improving this behavior to avoid such accidents. Perhaps we should include into the finished design everything that is inside the frame or visually overlaps it…
Thanks,
Dennis.