How to stop jQuery ajax from adding slashes to JSON string?
WordPress made the decision a long time ago to auto add slashes to all global input variables ($_POST, etc). They pass it through an internal wp_slash() function. The official recommended way to remove these slashes is to use their provided wp_unslash:
wp_unslash( $_POST['preset'] );**Note: It looks like this might be getting fixed in version 5.0, where they will be doing the wp_unslash for you when you request values from the global input variables.