i cannot give you any advice on REST with ACF. I also am not sure about setting an ACF field based on the post title, most people want to do it the other way around… set a post title based on an ACF field. I’m not sure of the timing here. I would probably do this using an acf/save_post hook instead of an update_value hook. https://www.advancedcustomfields.com/resources/acf-save_post/
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