Why is this async function returning empty on the first try, but then it returns everything?

Most modern JS libs that use async functions will return a promise, but will often not return a promise if the callback is supplied.

So assuming connection.query does return a promise, then the code below should do what your after..

async function insertarPracticas(turno_id, req, queryInsertarPracticas) {
    const res = await Promise.all(
        req.map(async (r) => {
            try {
                const row2 = await connection.query(queryInsertarPracticas, [
                    turno_id, r
                ]);
                console.log("Turnos detalle agregados " + r);
                practicasAgregadas += r + "-";
                console.log("practicas " + practicasAgregadas);
            } catch (e) {
                console.log(
                    "Error al insertar turno detalle (prácticas): " +
                        r + " " + error2
                );
                practicasNoAgregadas += r + "-";
            }
            return practicasAgregadas;
        })
    );

    console.log("en async " + res[0]);
    return res;
}

Remember, don't use a callback, or that could indicate to the lib that your not wanting a promise returning.

If for some reason the lib has not been updated for a long while, you will need to promisify the query..

In node, there is a nice utility called promisify https://nodejs.org/api/util.html#utilpromisifyoriginal that will make this bit easier.

eg.

const conQuery = util.promisify(connection.query);
const row2 = await conQuery(queryInsertarPracticas....

If the callback is not in the form (error, result) you would need to use a Promise constructor.. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/Promise , but looking at your code (error2, row2) => that shouldn't be required.