Fixed the multiple option when it is set false on belongsToMany selects

src/View/Helper/FormHelper.php

@@ -1288,7 +1288,7 @@ protected function _magicOptions($fieldName, $options, $allowOverride)
 
         if ($allowOverride && substr($fieldName, -5) === '._ids') {
             $options['type'] = 'select';
-            if (empty($options['multiple'])) {
+            if (!isset($options['multiple']) || $options['multiple']) {
                 $options['multiple'] = true;
             }
         }