All element positions in a BPMN XML file are coordinate based. In Signavio Process Manager the drawing area starts with the coordinates 0,0. Unfortunately, some other tools allow negative coordinates which lead to diagrams being partly outside of the drawing area. This also leads to the misplacement of some elements. To fix these diagrams, enlarge the drawing area by clicking on the yellow arrows at the border of the area until the whole diagram is visible. The problem should be fixed after you saved and reloaded the diagram.