I am using scatter series on top of a heatmap chart. The scatter series are used to display arrows between bubbles on the heatmap. To draw the arrows, I am using the following series configuration:

{
      type: 'scatter',
      enableMouseTracking: false,
      lineWidth: 1.5,
      lineColor: 'rgba(117,117,117,1)',
      dashStyle: 'dash',
      marker: {
        enabled: false
      },
      data: [[startBubble.x,startBubble.y],[targetBubble.x,targetBubble.y]],
      endMarker: 'url(#arrow)',
      dataLabels: {
        enabled: false
      }
    }

As you can see, I am using the endMarker property to draw the arrow heads.

I recently updated the Highcharts version from 6.2.0 to 9.2.2. In the versions up to and including major release 8, this endMarker property worked perfectly for my purpose. Since version 9, it does not seem to work anymore. There are no more arrow heads.

Did I miss an API change in the update notes, or is this feature not supported anymore?