Instead of creating a string try creating an array and check.
$posttypes_array = array();
foreach ($post_types as $post_type ) {
$posttypes_array[] = $post_type;
}
And then form the query as follows
$buildArgsAllQuestions = array( // Add out new query parameters
'post_type' => $posttypes_array,
//'post_type' => $posttypes_array,
'orderby' => 'date',
'order' => 'DESC',
);