Custom sorting on a column within "render" (multiple numbers)

I have a table that uses dataTables in AJAX mode. One of the columns contains one or two numbers (float). If there are two numbers, they are separated by a semi-colon. Example: 65.00;67.00

I want my users to be able to sort this column numerically, with this simple rule: if there are two numbers in a cell, dataTable should just ignore the second one. So in the example given above, just use 65.00 as a sorting value.

I'm trying to do that using the "render" option in columnDefs, but although the function is indeed triggered when I sort my column, the result is still sorted alphabetically (with "100" before "12", for instance).

Here's the part of columnDefs related to this column:

{
    "targets": 8, // using an array as source
    "render": {
        "display": function (data, type, row, meta) {
            if (data) {
                var r = '';
                var temp = data.split(';');
                if (temp[0]) {
                    r = temp[0];
                    if (temp[1] && temp[1] != temp[0]) {
                        r += '-' + temp[1];
                    }
                }
                return r;
            }
        },
        "sort": function (data, type, row, meta) {
            var r = 0;
            if (data) {
                console.log('sort:', data, '->', parseFloat(data));
                r = parseFloat(data);
            }
            return r;
        },
    },
},

When I trigger the sort (by clicking on the column header), the console.log is indeed printed to the console. Yet the resulting sort is still alphabetical.

What am I missing?