See eclipse-gmf-runtime/gmf-runtime#53, which fixes a memory leak in GMF Runtime's ConnectionEditPart. The fix is merged, but will not be available until a new 1.16.4 release is done (for which I have no fixed date yet).
Until then, since all our edge edit parts in Sirius inherit from the same AbstractDiagramEdgeEditPart, we can include a workaround on our side.

The missing removeNotationalListeners() which we will add on our side seems safe to be called mulitple times, so while it'd be better to remove our workaround once we move to GMF Runtime 1.16.4, it should not cause any actual issue if we don't.