Credit memo net due date is not correct

Hi experts,

In most cases the net due date of customers credit memos (posting key 11) is correct based on payment terms but sometimes itu2019s not (e.g. payment terms 30 days end of month but net due date is the same as document date).

Any ideas why?

I checked payment terms in customer master data and in FI and they are the same.

Thanks for your help,

Ben