Cannot create backward link from target node to source node

GoJS
1

I cannot create new link from target node to source node if source node already has created link with target node: https://youtu.be/gOJs9Ls4XGI.

It seems internal linking validation does not allow me to connect in this way, but I did not find suitable properties to resolve this validation.

This is a piece of layout where portId properties have declared:

$(
      Go.Node,
      'Auto',
      {        defaultAlignment: Go.Spot.Center,      },

      $(
        Go.Panel,
        'Vertical',
        {
          stretch: Go.Stretch.Horizontal,
          margin: new Go.Margin(0, STYLES.BORDER_WIDTH / 2, STYLES.BORDER_WIDTH / 2, 0),
        },

        $create(
    Go.Shape,
    {
      alignment: Go.Spot.Center,
      geometryString: 'F M144 80v96H48V80h96zM48 32C21.5 32 0 53.5 0 80v96c0 26.5 21.5 48 48 48H72v96c0 48.6 39.4 88 88 88h96v24c0 26.5 21.5 48 48 48h96c26.5 0 48-21.5 48-48V336c0-26.5-21.5-48-48-48H304c-26.5 0-48 21.5-48 48v24H160c-22.1 0-40-17.9-40-40V224h24c26.5 0 48-21.5 48-48V80c0-26.5-21.5-48-48-48H48zM400 336v96H304V336h96z',
      fill: 'white',
      width: 12,
      height: 12,
      strokeWidth: 0,
      background: 'transparent',

        portId: '',
        fromLinkable: true,
        toLinkable: false,
}
)
      ),

      // custom rounded rectangle acting as border
      $(
        Go.Shape,
        'CustomRectangle',
        {
          isPanelMain: true,
          strokeWidth: STYLES.BORDER_WIDTH,
          stroke: STYLES.COLOR,
          fill: null,
          spot1: new Go.Spot(0, 0),
          spot2: new Go.Spot(1, 1, 0, 0),
          parameter1: {
            borderRadius: STYLES.BORDER_RADIUS,
          },

          portId: '',
          fromLinkable: true,
          toLinkable: true,
        },
      ),
2

Check the value of the Diagram.validCycle property.

3

I have set to go.CycleMode.All and it works, thank you