If you use Advanced Custom Fields you may not realize it removes the ‘Custom Fields’ option from your Screen Options view and hides them entirely in the space they normally occupy below your posts/pages. That’s nice occasionally and a pain other times. Today it was a pain.
I figured this out one time before but it took me a while to re-find it at this github page so I’m leaving it here in case I forget again.
add_filter( 'acf/settings/remove_wp_meta_box', '__return_false' );
Yessssss! Thank you so much for this, it has been driving me crazy on one of my sites.