DataTables - scrollX causing squashed header

So I'm using DataTables with the scrollX parameter set to true, however it's causing the thead columns to collapse.

Note: the datatable is appearing inside a Bootstrap Modal as part of a react project.

How can I resolve this?

When I click on one of the columns, causing it to refresh, it fixes itself. Also, if I remove the scrollX it also doesn't collapse.

Initialisation code:

$('#item-search').DataTable( {
          ajax: {
            "data": {
              type: "map_items",
              map_id: this.map_id
            },
            "dataSrc": (r) => {
              console.log(r);
              return r.data;
            },
            type: "POST",
            url: "src/php/search.php"
          },
          autoWidth : false,
          columns: [
            {
              "data": "brand"
            },
            {
              "data": "name"
            },
            {
              "data": "quantity"
            },
            {
              "data": "measurement"
            },
            {
              "data": "type"
            },
          ],
          dom: 'rtlip',
          language: {
            "info": "Showing page _PAGE_ of _PAGES_",
            "infoFiltered": ""
          },
          lengthMenu: [ 1, 2, 3 ],
          pageLength: 1,
          processing: true,
          responsive: true,
          scrollX: true,
          select: {
            style: "multi"
          },
          serverSide: true
      });