I already had an issue with hunk refinement, that fixed by:
;; Since 27.1 it is enabled during font-lock. Need to disable explicitly.
(setq diff-refine nil)
So I checked /usr/share/emacs/27.1/lisp/vc/diff-mode.el.gz again and found:
(defcustom diff-font-lock-syntax t
So basically Emacs opens file behind in order to build proper syntax highlighting. That ...