Unable to Add Exceptions through code – array as a string errors

Hello,

I have a front-end form that creates posts (using gravity forms). I’m trying to code a function that assigns edit exceptions to a set of user ids using the following code:

// Change 1 in gform_advancedpostcreation_post_after_creation_1 to your form id number.
add_filter( 'gform_advancedpostcreation_post_after_creation_28', 'gf_post_pp_edit_multiselect', 10, 4 );
function gf_post_pp_edit_multiselect( $post_id, $feed, $entry, $form ) {
  
    // Checkboxes field id. Change this value to your field id number.
    $field_id = 2;
  
    // Get field object.
    $field = GFAPI::get_field( $form, $field_id );
  
    if ( $field->type == 'multiselect' ) {
        // Get a comma separated list of multiselect checked
        $selected = $field->get_value_export( $entry );
  		$reviewers = [];
        // Convert list of users to array with user ids.
        $reviewers = explode( ', ', $selected );
		$reviewerVals = array_values($reviewers);
    }
	
	GFCommon::log_debug( __METHOD__ . '(): reviewers => ' . print_r( $reviewers, true ) );
	GFCommon::log_debug( __METHOD__ . '(): reviewerVals => ' . print_r( $reviewerVals, true ) );
	GFCommon::log_debug( __METHOD__ . '(): selected => ' . print_r( $selected, true ) );
//	reviewers => Array
//	(
//    [0] => 7
//    [1] => 8
//    [2] => 97
//	)
	
	// enable all members of this group to read pages which are in the specified term
$args = array(
	'operation' => 'read',
	'mod_type' => 'additional',
	'for_item_source' => 'post',
	'for_item_type' => 'post', 
	'via_item_source' => 'post',
	'item_id' => $post_id, 
);
	
GFCommon::log_debug( __METHOD__ . '(): ppc_arguments => ' . print_r( $args, true ) );

ppc_assign_exceptions ( $reviewerVals, 'user', $args );
//tried with $reviewers, $reviewerVals, $selected - no luck!
	
	GFCommon::log_debug( __METHOD__ . '(): ppc_assign_exceptions output => ' . print_r(ppc_assign_exceptions ( $reviewers, 'user', $args )) );
	
}

Could you kindly help me?

The errors I get are:
Warning: array_keys() expects parameter 1 to be array, string given in ".../publishpress-permissions/classes/PublishPress/Permissions/DB/PermissionsUpdate.php " on line 183

And so on and so forth.

What am I doing wrong? How am I supposed to call the ppc_assign_exceptions() function for a set of users if I have their ids?