The U.S. Supreme Court expressed interest April 22 in a case involving Putnam Investments LLC’s 401(k) plan by asking the federal government to file a brief expressing its opinion on the dispute.

The case asks which litigants—the fiduciaries of employee benefit plans or the people covered by those plans—have the burden of proving in court that alleged fiduciary misconduct caused plan losses. This is the third time in four years the Supreme Court has invited the solicitor general to file a brief expressing the government’s opinion on this question.

It’s also the second time this month the justices have sought...