Putnam Investments LLC overstated how much the circuits are split on interpreting the Employee Retirement Income Security Act, Putnam employees told the U.S. Supreme Court.

The justices shouldn’t second-guess a First Circuit decision reviving a challenge to the investment company’s 401(k) plan, the employees said in a March 14 brief. The circuit split Putnam cited in its petition for Supreme Court review is “more imagined than real” and doesn’t apply to the dispute at hand, the employees said.

The decision the company wants overturned placed the burden on Putnam to show that its alleged retirement plan missteps—offering workers expensive and...