I did try the same thing inserting on nextgen gallery in wysiwyg acf field it works but in flexible content it doesn’t work.
Sorry I did put it on the wrong wysiwyg built-in wp not the acf wysiwyg field, I did recheck an its doesn’t render the gallery.
I think NextGen has custom code/tag to identify and render the gallery. Its not using a shortcode anymore. 🙁
Hope to hear from you end.
Thanks