This issue is labeled with difficulty: easy .
@fvales Would you like to debug it by yourself? This is a quicker way to get your problem fixed. Or you may wait for the community to fix.

Please have a look at How to debug ECharts if you'd like to give a try. 🤓

selectedMode is based on the name of the series. It is mainly used in 1*1 data dimensions such as legends and bar charts.

Cells with the same name on the heat map will be selected at the same time.

In order to avoid modifying the source code, you can use mouse events to listen to params and highlight the style according to the data of the selected cell.

option = {
  .....
  series: [
  {
    name: 'Punch Card',
    type: 'heatmap',
    selectedMode: false, // disable selected mode
    data: data,
    itemStyle: {
        emphasis: {
          borderColor: 'black',
          borderWidth: 2
        }
    }
}]}
var previousSelected = null;
myChart.on('click', function (params) {
  // clear previous selected
  if (previousSelected) {
    myChart.dispatchAction({
         type: 'downplay',
         seriesIndex: 0,
         dataIndex: previousSelected.dataIndex
     });
 }
  //highlight current selected
    myChart.dispatchAction({
        type: 'highlight',
        seriesIndex: 0,
        dataIndex: params.dataIndex
  });
  previousSelected = params;
});

option && myChart.setOption(option);