Edges are drawn as lines from the point where the path leaves the origin node up to the point where it reaches the destination node. In the absence of any arrows, that path is drawn using a butt line cap, so a straight edge always becomes a rectangle when drawn. This is not a problem if the bounding box of the node has the same thickness as the edge. But drawing an edge thicker than this bounding box and not orthogonal to the nodes will result in rather ugly line terminators:
This should be improved. Perhaps one of the best ways to fix this would be to always draw the edge up to the center of the node, but to omit the part inside the node using clipping. That way, the shape of the end of the edge would follow the shape of the node.