For thousands of manufacturers across China, it’s déjà vu—with implications for the country’s fragile economy. Earlier this year, after President Trump raised tariffs on Chinese goods to 145% in April, American customers of Alan Chau’s toy factory in southern China abruptly froze orders, sparking a cash crunch ttohat brought his business to the brink. So it came as a relief when the U.S. and China reached a trade truce weeks later in mid-May, rolling back most of their tariffs on one another—and allowing Chau to resume shipping his products again. Now, less than six months later, prohibitively high tariffs could...