A South Korean regulator fined the country’s largest e-commerce platform, owned by U.S.-listed Coupang Inc., a record 624.7 billion won ($409 million) for a wide-ranging cyber-intrusion that escalated into a diplomatic tiff with the US. The Personal Information Protection Commission’s fine for Coupang Corp.—the company’s South Korean entity—is the biggest-ever levied by the country over a personal data breach since a 134.8 billion won penalty on SK Telecom Co. last year. Under Korean regulations...