When we have ports which the user can draw/create connectors from, we sometimes find that when the connector is created, it isn't attached to the port.
It seems to be that this happens when you click on the very edge of the port. The control recognises that you want to draw a connector from the port but fails to attach the connector to the port.
In the attached sample, zoom right in to the port and hover over it. Now move your mouse over the blue line that appears around the port. Click and drag to draw a connector. On some occasions the connector will not be attached to the port.
It seems to happen more often if you put your mouse over the bottom side of the blue square.
We aren't normally zoomed in so much. But this can happen when zoomed out too.