55 sailors is a small crew for that sized sub.
The submarine surely wouldn’t have ran out of oxygen if it’s a Shang class vessel, and therefore nuclear. Its machinery could keep on supplying its crew with oxygen for *years.* Maybe one of China’s own mines blew it up? Or it got tangled up and sank past crush depth into the deep ocean abyss? What are the chances its reactor(s) malfunctioned, taking its oxygen generating equipment offline? Disasters sometimes result from more than one system failing at the same time, aka “Murphy’s Law.”