Hello Nicolas,
it's really strange.
I've checked it another time some minutes ago and, on my machine, this
works.
I've used the following command to do the test :
for emacs 25.3
#+begin_src sh
emacs -Q -L lisp -L ~/.emacs.d/elpa-25/htmlize-20180412.1244 -l ~/.emacs.d/elpa-25/htmlize-20180412.1244/htmlize-autoloads.el testing/examples/test.org
#+end_src

for Emacs 26.1 (934bb475b9)
#+begin_src sh
emacs -Q -L lisp -L ~/.emacs.d/elpa-25/htmlize-20180412.1244 -l ~/.emacs.d/elpa-25/htmlize-20180412.1244/htmlize-autoloads.el testing/examples/test.org
#+end_src

And I still obtain the expected results : see the text attached file
(encoded in utf-8-unix).