my trading account balance is negative

Some brokers have systems that clear the negative balance in your account immediately while others wait till they run their overnight reconciliation before the negative balance is cleared.

The ones that let the negative balance sit in your account till midnight I would say are risking regulatory sanction.

Negative Balance Protection doesn't stop your account from going negative during an open trade, it only ensures you don't owe the broker money after a trade closes.

So the key question is, is your balance negative while a trade is still open, or after it closes?

If it's after closing, raise it with your broker immediately. That would be a clear violation of NBP rules.