I have tested dropdown in subgrid detail, it works as expected by the code.
Refer dropdown sample, OR update the error code / screenshot via email ([email protected])

$col["edittype"] = "select"; // render as select
$str = $g->get_dropdown_values("select distinct client_id as k, name as v from clients");
$col["editoptions"] = array("value"=>$str);
$col["formatter"] = "select"; // display label, not value$cols[] = $col;
$cols[] = $col;