User has applied discounts to many invoices, but the Project Profitability report does not show them under the Credits & Write-Offs column.
This is the expected behavior in Core. The Credits & Write-Offs column shows the total from payments of type Credit or Write-Off. However, discounts that are applied to invoices during their creation are excluded from this calculation. Check out this article to know more about credit and write-off payments: Payment method: Credit vs. Write-Off.