Yes, it was too obvious. I mentally smacked myself in the head when I saw it because like everyone else I’ve been using an acf/save_post action on pretty much every site I’ve built since starting to use ACF, quite a few years ago now. Simply because I wanted to make the featured image required and be able to enforce specific image sizes for different sites/post types.
The only real problem with using this method is that get_fields() will not return this field. ACF ignores all fields that start with an underscore. These fields are also hidden by WP. But since I’ve never used get_fields() it works out fine for me.
