I like this one, too:
Exactly. The question here is when it was clear in the art community that this was a rotated Matisse. From
Fort Wayne Museum publication in 1998it is clear it was known by 1998. Carefully looking at that publication though reveals that the author (Ann Gibson) didn't know about the relation between the paintings before she wrote the article, because she says in footnote 36 (page 52) that it was Sachi Yanari (the curator of the museum) that pointed this out to her. She even mentions the date of the letter (March 6, 1997) and the fact that she found a copy of the Matisse painting in the file of the Hirschhorn.
By explicitly crediting Yanari, she makes clear that the relation between the paintings was not widely known by the time. And this was 34 years after the painting was made. So yes, it was probably known "for decades" that Watusi was inspired by Matisse but the fact that it was actually a copy was not. That only came out in the open in 1998.
Does that make it a fraudulent copy? We have to be careful here, but it definitely does not look good.