If a system glitch allows untraceable withdrawals or transfers and I report it but it remains unfixed, is using it illegal?

There are always two parties in the authorization and circulation of money. If you hack or take advantage of the system such that you unilaterally give yourself money, the counter party claim is the main issue here. As there was no authorization in the issuance and acceptance, the money could very well be reversed.

Whether or not it is the system' fault, if you are conducting a transaction on which you have no authorization, it is then illegal. Period.