Thanks for your response James. I’ve had much more consistent results as of late. I’m hoping the issue was theme related and has been resolved. This support request can be closed for now.
I too was unable to get this filter working within in my child-theme’s functions.php, but putting it in a plugin worked as well for me. Any ideas why this might be the case?
You could use the sanitize_text_field function.
https://codex.wordpress.org/Function_Reference/sanitize_text_field
echo sanitize_text_field( get_field( 'acf_textfield' ) );