HONG KONG—When a small group of workers spent 27 hours in early October locked inside the world’s biggest iPhone plant, it seemed little more than another temporary confinement in China’s continuing war against Covid-19. But the outbreak persisted, and on Monday, days after saying operations were stabilizing, Foxconn Technology Group cut its outlook for the current quarter. The company said it was locking down eight of the 11 dormitory blocks at the plant in the central Chinese city of Zhengzhou as part of a government-directed plan to end the outbreak.