The same happens with UML state machine diagrams. Unpredictable and quite annoying behavior. This had been reported to Sparx as bug already for version 14 and is still not fixed with version 15 RC6. Try dragging the any side of a state element towards the adjacent side of the diagram frame, the result looks quite funny ;( . To that extend that I still prefer to use EA 13.5 for state machine diagrams that have to go into specification documents. Another annoying feature is that diagram frames don’t use transparent backgrounds anymore. This was useful to align connectors to the grid. I might have missed a hidden feature setting to disable diagram frame backgrounds.
