Mr. Poehling’s allegations, if true, could help explain why insurers are staying in the Medicare Advantage program even as they pull out of the Affordable Care Act exchanges in some states: Medicare Advantage offers a way to get extra money from the federal government.

When a whistle-blower succeeds in recovering money for the government, the False Claims Act calls for him or her to receive a percentage. Many whistle-blower cases fail to reach that point, but when the Justice Department joins a case, in general, the odds of a recovery go up.

Already the Justice Department has declined to intervene in some smaller whistle-blower cases with similar allegations. But in March, it did say it would join a whistle-blower suit filed by James Swoben, a former data manager of SCAN Health Plan, accusing UnitedHealth and several other companies of cheating Medicare Advantage by looking improperly for ways to raise people’s risk scores.

In 2016, the United States Court of Appeals for the Ninth Circuit vacated a lower court’s decision to throw out Mr. Swoben’s case. After reviewing the allegations, Judge Raymond C. Fisher wrote, “We do not see how a Medicare Advantage contractor who has engaged in such conduct can in good faith certify” that the diagnosis codes it reports to the Centers for Medicare and Medicaid Services “are accurate, complete and truthful.”

That ruling did not decide Mr. Swoben’s case, but merely sent it back to a district court to be adjudicated. His case and Mr. Poehling’s case are both now being handled by the United States District Court in Los Angeles.

Meanwhile, UnitedHealth has sued the Centers for Medicare and Medicaid Services, seeking to vacate a 2014 rule that requires insurers to make sure the diagnoses they report to the government are borne out by what is in people’s charts, and imposing penalties for overstatements. UnitedHealth argues that this rule unlawfully departs from the program’s statutory mandates requiring “actuarial equivalence” with the traditional Medicare program.

“That case could provide further clarity on the program rules,” Mr. Burns of UnitedHealth said. He added that the government seemed to be trying to delay so that the two whistle-blower lawsuits could go first.