I heard Brendel perform the last 3 Beethoven sonatas back in 1995 in
Montreux, Switzerland.  I went with a good friend of his, and I for the
first time heard the 'jazz' element that I think you are referring to.
I was a bit perturbed by it, and his friend (a well known Swiss concert
pianist) assured my that I heard it performed correctly and that no
offense should be implied.  Haven't heard it played that way since, and
on Kovacevich's earlier recording he is more straightforward with his
rhythmic fluctuations.

JPrevite