I was reading reports [1,2,3,4] about a recent Air China flight where one of the pilots was allegedly smoking and, in an effort to prevent the smoke circulating to the cabin, accidentally de-pressurised the aircraft. The passenger oxygen masks were deployed and the crew performed an emergency descent. Eventually the problem was rectified and, after the cabin re-pressurised, the plane climbed again and completed the flight.
The bolded parts of this story are what I'm asking about. If the oxygen masks have been deployed and the plane returns to cruising altitude, then there is no supplementary oxygen available for passengers in the event of a second de-pressurisation incident. This seems unsafe. (I recall reading that the oxygen supply only lasts 12 minutes and is intended only to last for the duration of an emergency descent.)
What are the rules around this situation? Can an aircraft safely/legally climb again after passenger oxygen masks have been deployed?