We have been using the agreement module for many years and still we have the "bug" that payment terms is empty when we make invoices from agreement. Then I have manually to fill it in for every invoice and that is frustrating.
If you are using an old version, then it used to paste a specified salesman into the "Payment terms" field, I can see it is fixed in the 8.5 2021-08-28 version (maybe earlier).