Billing a Sales Order with Discount item setup to have a numeric value displays the Discount in a percentage format on the Cash Sale or Invoice form.

This is an expected NetSuite behaviour if "Convert Absolute Discounts to Percentage" preference is enabled.  To verify the said preference, the user can navigate to Setup > Accounting > Accounting Preferences > Order Management tab > Invoicing section.  The "Convert Absolute Discounts to Percentage" checkbox must have been marked.

Using this preference, when a sales order that includes a currency-amount discount is billed on multiple invoices, the discount is applied evenly to all bills for that order instead of the entire discount being applied to the first bill.

For example, you use Advanced Billing to enter a sales order for $1000 that includes a line-item discount of $100. The sales order will be billed in 10 monthly installments of $100.

When you bill the order, the first invoice shows a 10% discount in the amount of $10.00 and each of the next nine bills will show a 10% discount in the amount of $10.00.

Note: The user can anytime turn on/off this preference depending on their requirement.