Sorry for delay in reply.
Hmm..I don't know why is this happening .Because if you have done the change correct then it should just add that constant .Also please note we are not modifying the UDF anytime,but just input to the UDF.
I think you should raise an ticket to SAP for this.
Also, if possible send the correct you made, please? for both the main header and the field.
Also undo the change and see what error you are getting and after change what you are getting in dynamic config.
Thanks