BERLIN — Volkswagen has been struggling for a year to repair the damage caused by a scandal over its cover-up of diesel emissions, promising honesty and transparency. Now historians are accusing the company of reverting to secretive ways on a different subject: the Nazi past of German automakers.

Over the past 18 years, Volkswagen became something of a pioneer in revealing the company’s employment of thousands of forced laborers during World War II. But it has abruptly parted ways with the company historian who helped make that possible.

When the historian’s contract abruptly ended this week, an angry open letter signed by 75 prominent German academics accused Volkswagen of a vindictive punishment. The historian, Manfred Grieger, and the company have declined to comment on the circumstances behind his departure, citing a mutual agreement to end his contract.

But the mystery over precisely why Mr. Grieger left — and whether he was dismissed — has complicated Volkswagen’s effort to regain public trust, and risks stirring up a dark chapter in company history.