The hum of solar panel factories in this steamy island city doesn’t sound like much. But to American trade officials, it’s the muffled noise of rules being bent — or broken.

In recent months, Batam, a duty-free Indonesian enclave a short ferry ride from Singapore, has become a key waypoint in a convoluted global shuffle. Chinese solar manufacturers, facing stiff U.S. tariffs, are quietly assembling their equipment in Indonesia and slipping their products into the American market, tariff-free.

It’s an elegant workaround. But it may not last long.