How to prevent highcharts from shortening labels with ellipsis

Set proper styles for that labels, see API.

Example:

xAxis: {
    labels: {
        style: {
            textOverflow: 'none'
        }
    } 
}

Try override the default behavior by yourself.

  xAxis: {
    labels: {
      useHTML: true,
      formatter() {
        let label = this.value;
        let title = this.value;
        let style = `text-overflow: ellipsis; overflow: hidden;`; // <- YOUR OWN STYLE
        return `<div style="${style}" title="${title}">${label}</div>`;
      }
    },
  },

Style reference: https://makandracards.com/makandra/5883-use-css-text-overflow-to-truncate-long-texts