I have this problem too, and it is incredibly frustrating.
It seems to happen after manually dragging a visual to a new position and then clicking the up/down arrows to set the size and/or position. After it happens there is no easy way to correct because the mobile editing mode does not allow the user to see the visual in order to reselect and move it to a new position or delete it etc. It looks like there is some kind of logic in the position and size entry fields that is trying to prevent the user from entering a size that would take the visual off the page, but it is not working. I would personally prefer it if the editor did not try to "help" me and it should be my problem if I choose to locate the visual outside of the visible area.