In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2020-05-17, modified by Debian built on x86-csail-01

To see the problem, launch dired-diff on the attached files (to do that,
open dired, and hit = on one of the file entries)

You see that the diff chunk differs for whitespace and case.  Try first
C-b (refine diff) and then C-w (ignore whitespace).  It is not possible
to have them both active at the same time, while it should.

The same happens for vc-diff