Betting against U.S. government debt this year is turning out to be a fool's errand. Just ask Wall Street's biggest bond dealers.

While the losses that their economists predicted have yet to materialize, JPMorgan Chase & Co., Citigroup Inc., and the 20 other firms that trade with the Federal Reserve began wagering on a Treasuries selloff last month for the first time since 2011. The strategy was upended as Fed Chair Janet Yellen signaled she wasn't in a rush to lift interest rates, two weeks after suggesting the opposite at the bank's March 19 meeting.

The surprising resilience of Treasuries has investors re-calibrating forecasts for higher borrowing costs as lackluster job growth and emerging-market turmoil push yields toward 2014 lows. That's also made the business of trading bonds, once more predictable for dealers when the Fed was buying trillions of dollars of debt to spur the economy, less profitable as new rules limit the risks they can take with their own money.