Hi @nigel-rodgers
The field disappeared because there’s an error in your code, but you don’t have the debug mode enabled. So instead of showing the error message, WordPress was just showing nothing. This page should give you more idea about it: https://codex.wordpress.org/Debugging_in_WordPress.
The issue you had was because you had a different function name between the filter and the function itself. The filter uses “make_hidden_post_id_readonly” while the function is “make_field_readonly”. Kindly change it so they have the same function name.
I hope this helps 🙂
