The WYSIWYG field can support shortcode because it will implement the do_shortcode() function when the value is returned. Shortcodes are officially used by WordPress and other plugins, so ACF is supporting it.
The problem with the tag is that NextGEN will only watch the post content for its tag and apply a function to it. Unfortunately, ACF doesn’t know which function it’s. Even if I check their code, it will be hard for me to find it and apply it correctly. That’s why asking them regarding this topic would be a better choice for you as they are the one who know their code.
I hope this makes sense 🙂
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