Zero is a real cap: it blocks new charges. It is different from an unset limit, which allows spending subject to the team's other safeguards. Already-paid service can continue until the paid window ends even after a new cap blocks its next charge.
If you intended to remove the cap, use the relevant Remove limit action or empty limit field and save the change. Review customer and project caps as well as the number's own cap. If a saved change is not reflected, report the value and time to support. Do not repeatedly reconnect to bypass a budget.