Nomura Holdings Inc.’s chief executive officer was having a bumper inaugural year in charge — until a U.S. family office spoiled the party.

Just days before Kentaro Okuda’s first anniversary as head of Japan’s biggest brokerage, the company warned of a "significant” loss from an unnamed U.S. client. That’s tied to the massive unwinding of leveraged bets by Bill Hwang’s Archegos Capital Management, according to people familiar with the matter.

The debacle triggered a record 16% drop in Nomura’s shares Monday, wiping $3.5 billion from its market value and threatening a turnaround executives had hoped would herald a new era of more sustainable profits.