This happens if the Sales Order has a Billing Schedule with a different Payment Term from the Terms set on the Sales Order.
.Setup > Accounting > Accounting List > Terms > edit Net 45 as Preferred.
. Edit a Customer Record > Financial tab > Terms field > choose Net 30
. Go to List > Accounting > Billing Schedules > Recurrence Payment Terms = Net 10
. Create a Sales Order we will notice that the Customer Default Term – Net 30 is populated on the Sales Order’s Terms field.
. Make invoice
Notice that the Terms field value changes to Net 10.
To fix this:
1) Edit by going to List > Accounting > Billing Schedules > edit Billing Schedule > Set Recurrence Payment Terms to blank > Save
2) Edit the Sales Order > Save (without any changes)
3) Create an Invoice