I think the issue is that you only add the custom field values from the Formidable form. ACF saves two entries in the database, the value and the reference. The reference is needed to let ACF know which field you want to update or get the data.
To fix it, kindly update the custom field value by using the update_field() function. Don’t forget to use the field key instead of field name on that function.
I hope this makes sense 🙂