But have you already started the withdrawal process?
Itch only validates your interview when you request your first withdrawal (there's a $3 fee to validate the interview, which is deducted from your first withdrawal). Before that, the amount shown is only an estimate, not the actual amount, which is why it shows a 30% withholding.
When you complete the withdrawal process, a full report is generated showing all the exact amounts, and in my experience, I've never had a problem.