All of this has already been confirmed by MSFT that this is a bug but they said they would not fix it and suggested I post it here to get votes. (Thanks MSFT - very helpful...) MSFT ticket # 2510270040008066 for reference.

Currently, When a purchase order is removed from a vendor invoice, the payment terms are also removed at the invoice level, causing the due date to match the invoice date incorrectly. This can cause early payment to vendors and other possible payment issues if it's not caught.

The payment terms should be kept on the invoice, it should follow whatever the vendor payment terms are and not be removed. This is for sure a bug that MSFT confirmed happens in a standard environment with no custom code.