I have noticed that when using custom order fields with type checkbox, we have tags which returns System.String "yes/no" values instead of boolean "true/false"
in this situation @GetBoolean("FragtmandAgree.Clean") will always return "false" value.
So, we have to use unnecessary conversations when working with custom checkbox fields in order page. I think orderline, product, etc, custom checkbox fields have usual "true/false" output.