That is because the AI doesn't have enough context knowledge to distinguish prominent signatures from stylistic flourish or a thing being represented in the pai

That is because the AI doesn't have enough context knowledge to distinguish prominent signatures from stylistic flourish or a thing being represented in the painting. The generated signatures themselves are usually very different, though in some cases they can come very close to the original artist signature when properly prompted for artists with "big" signatures. Better image part tagging will solve this.