Dropdown resets on autopostback C#

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ASP.NET

C#5.0

I have a dropdown that populates with the selection of another dropdown.
I first clear this dropdown and then populate it, and I have several more of these that are linked to other dropdowns, and the others work great except the last one, which on certain selected indexes, it resets to the first value.
I have set autopostback to true, but don't know what else to do, and I am not getting any result from mysql db.
What I am trying to do with this last dropdown is return the values of a field in a db table, if they are found, depending on multiple criteria found in the where clause.

What I have tried:

String connString = System.Configuration.ConfigurationManager.ConnectionStrings["web"].ToString();
        conn = new MySql.Data.MySqlClient.MySqlConnection(connString);
        conn.Open();
        queryStr = "";
        queryStr = "SELECT * FROM avance_actividad where idProyecto='" + idproy + "' and idObjetivo='" + obj.SelectedIndex + "' and idResultado='" + res.SelectedIndex + "' and idActividad='" + act.SelectedIndex + "'";
        cmd = new MySql.Data.MySqlClient.MySqlCommand(queryStr, conn);
        //con.Open();
        var reader = cmd.ExecuteReader();
        while (reader.Read() && reader.HasRows)
        {
            // here could be problems if database value is null
            es.Text = reader["estado"].ToString();
            com.Text = reader["comentario"].ToString();
            por.Text = reader["porcentaje"].ToString();
        }
        conn.Close();