[AUCTeX] This code works in some TeX buffers and not in some others. Any ideas?

[Marcin Borkowski]

Hi all,

I don't want my tildes to stand out too much, so I have this in my
init.el:

(add-hook
'TeX-mode-hook
(lambda ()
(font-lock-add-keywords
nil
'(("~" . 'font-latex-sedate-face)))))

In some LaTeX buffers it works, in some not. Why this might be? How to
even debug this?

(AUCTeX auctex-11.89.1, from Elpa)

Best,