– Sir Michael Francis Atiyah
 e.g. https://en.wikipedia.org/wiki/Atiyah%E2%80%93Singer_index_th... (though not the best for the more physics minded)
Interesting discussion at:
And mathematicians taking him seriously:
Our ability to preserve bodies far longer than mind is a major problem for everyone.
People were mostly just sad that journalists were hyping up an obvious case of age-related cognitive decline from a brilliant Mathematician.
And to be clear, Atiyah's last work wasn't speculation – it was completely off, in the "not even wrong" category. Unfortunately, that's pretty common as people get older, but there's no reason to publicize it.
Are there other examples of this in Mathematicians? Nash and Godel come to mind, but both of them had non-math-related issues.
this is my theory anyway.