Very simple datatable program: -

new DataTable('#branchTable', {
    ajax: {
        "url": "/api/Service_Arm",
        "type": "GET",
        "dataSrc": "data"
    },
    columns: [
        { "data": "service.name" },
        { "data": "name" },
        {
            "data": "id",
            "render": function (data, type, row) {
                return `
                        <a href="/Admin/Service_Arm/Edit?id=${data}"><i class="btn btn-primary fa-regular fa-pen-to-square"></i></a>
                        <a href="/Admin/Service_Arm/Details?id=${data}"><i class="btn btn-success fa-solid fa-circle-info"></i></a> 
                        <a href="/Admin/Service_Arm/Delete?id=${data}"><i class="btn btn-danger fa-solid fa-trash"></a>`;
            }
        }
    ],
    order: [[0, "asc"]], // Sort by the 2nd column  in ascending order
    rowGroup: {
        dataSrc: 0
    },
});

The data returns correctly, and the table forms correctly, but no group is found.

The data feed from an SQL Table is:-

{
    "data": [
        {
            "id": 1,
            "name": "----- Select Your Service ---",
            "serviceId": 3,
            "service": {
                "id": 3,
                "name": "British Army",
                "service_TypeLocations": [],
                "service_Countries": [],
                "postings": []
            },
            "branch_1s": [],
            "postings": []
        },
        {
            "id": 2,
            "name": "Household Cavalry",
            "serviceId": 3,
            "service": {
                "id": 3,
                "name": "British Army",
                "service_TypeLocations": [],
                "service_Countries": [],
                "postings": []
            },
            "branch_1s": [],
            "postings": []
        },
        {
            "id": 3,
            "name": "Line Cavalry",
            "serviceId": 3,
            "service": {
                "id": 3,
                "name": "British Army",
                "service_TypeLocations": [],
                "service_Countries": [],
                "postings": []
            },
            "branch_1s": [],
            "postings": []
        },
----------------and on and on---------------

If I populate the table from the razor page controller and remove the ajax call it all works.
But it finds no group when I instigate the Ajax call