Highcharts Demo - a constant series line is not plotting when using linear gradient colour.

HTML

<script src="https://code.highcharts.com/highcharts.js"></script>
<script src="https://code.highcharts.com/modules/exporting.js"></script>
<script src="https://code.highcharts.com/modules/export-data.js"></script>

<div id="container" style="min-width: 310px; height: 400px; margin: 0 auto"></div>

JavaScript

Highcharts.chart('container', {
    chart: {
        type: 'line'
    },
    title: {
        text: 'a constant series line is not plotting when using linear gradient colour.'
    },
    legend: {
        layout: 'vertical',
        align: 'left',
        verticalAlign: 'top',
        x: 150,
        y: 100,
        floating: true,
        borderWidth: 1,
        backgroundColor: (Highcharts.theme && Highcharts.theme.legendBackgroundColor) || '#FFFFFF'
    },
    
    "colors": [
    {
      "linearGradient": {
        "x1": 0,
        "y1": 0,
        "x2": 1,
        "y2": 0
      },
      "stops": [
        [
          0,
          "rgb(35,190,250)"
        ],
        [
          1,
          "rgb(51,223,188)"
        ]
      ]
    },
    {
      "linearGradient": {
        "x1": 0,
        "y1": 0,
        "x2": 1,
        "y2": 0
      },
      "stops": [
        [
          0,
          "rgb(250,79,168)"
        ],
        [
          1,
          "rgb(156,120,229)"
        ]
      ]
    },
  ],
    
    xAxis: {
        categories: [
            'Monday',
            'Tuesday',
            'Wednesday',
            'Thursday',
            'Friday',
            'Saturday',
            'Sunday'
        ],
        plotBands: [{ // visualize the weekend
            from: 4.5,
            to: 6.5,
            color: 'rgba(68, 170, 213, .2)'
        }]
    },
    yAxis: {
        title: {
            text: 'Fruit units'
        }
    },
    tooltip: {
        shared: true,
        valueSuffix: ' units'
    },
    credits: {
        enabled: false
    },
    plotOptions: {
        areaspline: {
            fillOpacity: 0.5
        }
    },
    series: [{
        name: 'John',
        data: [2, 2, 2, 2, 2, 2, 2]
    }, {
        name: 'Jane',
        data: [1, 1, 1, 1, 1, 1, 2]
    }]
});