Custom string not working

Hi,

I use pie register for registration. I need to rename a file using a pie register field (pie_text_4).

So I enabled custom string {cf:pie_text_4} and set as renaming rule {custom_string} but the string isn’t resolved and the file is renamed as {cf:pie_text_4}.*

I the created a custom field with ACF with the same name. The ACF fiel gets the same value as Pie Register field but renaming is still not working.

Can you help me please?

Thanks

Kind regards

AT