Hi @cc
That’s weird. I’ve just tested that code and it was working great on my end. It’s possible that there’s a conflict on your site. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
I’m sorry I didn’t notice that you use the free version. I’m afraid the “acf/upload_prefilter” is only available in the PRO version. In this case, you need to use the wp_handle_upload_prefilter, but you need to use extra logic to determine if the images are posted from a ACF or not.
Thanks 🙂
