TOKYO – The Japanese yen’s sharp decline may be producing an unexpected loser: the US Treasury Department. One of the most intriguing mysteries of the last three months is this: Why is Japan, the biggest foreign holder of US Treasury securities, placing so many “sell” orders? In three months, Japanese institutional managers have dumped a cool $60 billion of US paper. Granted, that is a drop in the proverbial bucket considering Tokyo holds a $1.3 trillion stack of Washington’s IOUs. But the magnitude of the selling is getting increasingly hard to ignore. The most plausible explanation is the yen’s 13% drop so far this year. That fall complicates the economics of loading up on US debt at a moment when US inflation is at 40-year highs. That’s despite the negligible 0.22% return investors receive on 10-year Japanese government bonds.