It was Monday morning in Beijing, and Jack Ma had been summoned to a conference room at the China Securities Regulatory Commission just days before he was set to take Ant Group Co. public in the biggest stock-market debut of all time. When the bureaucrats finally turned up, they skipped over pleasantries and delivered an ominous message: Ant’s days of relaxed government oversight and minimal capital requirements were over. The meeting ended without a discussion of Ant’s IPO, but it was a sign that things might not go as planned.