Hi @marcgott
I can’t explain the ‘_nnnn_’ either, but it would be worth a shot to do a search for the string ‘_nnnn_’ within your entire WP folder.
Does it appear anywhere?
Perhaps you could also search for all ‘acf/update_value’ refernces and debug those blocks of code to find this strange $post_id value
Good luck, let me know what you find,
Thanks
E