The U.S. government has been issuing more debt, but it’s not getting more foreign buyers in the door. As a result, U.S. investors have so far financed all of this year’s increase in the federal government’s borrowing.

Foreign holdings of the debt have remained essentially flat, though the government’s borrowing has risen by $500 billion, giving foreign investors the smallest share of U.S. government debt since 2003. Even as yields on Treasury securities have risen to multiyear highs, foreign demand for debt at government bond…