I recall reading an essay arguing the real reason for US entry into World War One was to protect our war loans and econoimc ties to the Allies. The historian was somewhat well received in the 1930s, but later became anti-Semitic and a Holocaust denier and so was discredited. It made me wonder if any similar scandals have happened more recently.
Someone who comes to mind is Stephen Ambrose, who was a major American historian in the late 20th century (he died in 2002), and wrote extensively about World War II, with the HBO series Band of Brothers being based on a book he wrote. He also wrote an Eisenhower-approved Eisenhower biography, and co-founded the National D-Day Museum.
But late in his career he was hit with evidence that he had heavily plagiarized other historians' works, and had often been fast and loose with details and facts. We don't have a very modern thread on that topic, but I did find this thread with a discussion on Ambrose by u/Georgy_K_Zhukov and u/Rittermesiter.
I'm not sure which historian is being referred to in the OP, but that does remind me a bit of someone else, namely David Irving. Irving never was a professional academic historian, but his initial work was considered well-sourced, and was well received, such as The Destruction of Dresden (which Vonnegut cited approvingly by name in Slaughterhouse Five). However, it became increasingly clear especially in his later work that he was a Nazi sympathizer and Holocaust denier, and he was successfully prosecuted for the latter in 2000. Even his earlier work suffers from his selective use and misuse of sources and this bias, and that's become much more obvious with hindsight. For more in Irving see this roundup of comments, via u/DanKensington.
Stephen Ambrose was first publicly accused of plagiarism in 2002, and eventually several of his works were found to have taken passages verbatim from other authors without proper citation. That’s a very different sort of case, however, as it is only discrediting to him, and not to his presentation of history nor his historiography. It’s also worth noting that although he was an academic historian, he was most known for writing pop-history. Interestingly, his plagiarism occurs in both his academic and lay-oriented work, although his dissertation is the only academic work I’ve seen accused directly.
Additionally, after the plagiarism story broke, Ambrose was accused in several places of errors of factuality and an unwillingness to make corrections when he was informed of them. These allegations are harder for me to confirm directly, but as an example many D-day troop carrier pilots have objected to his characterization that they were untrained for the day of invasion, claiming they trained for over a year.
http://hnn.us/articles/504.html provides a good summary, although many of the linked news articles are no longer available from the urls listed.