Why is the Discard button on the Payables invoice line grayed out?

The invoice has Qty Rec hold and needs revalidation.

There are 5 invoice lines: 3 item lines, 1 tax line, and 1 withholding tax line

The withholding tax line was created at the time of invoice validation.

After invoice validation,

on the 3 item lines and the tax line > Discard button is grayed out

the withholding tax line > Discard button is enabled.

Question: Why is the Discard button on the 4 lines grayed out?