just tested again with my current setup of 26.1 and indeed it seems the
issue is resolved.

Am Mo., 30. Nov. 2020 um 12:46 Uhr schrieb Lars Ingebrigtsen <larsi@gnus.org
>:

> Roman Decker <roman.decker@gmail.com> writes:
>
> > whitespace-mode seems to ignore file- and dir-locals when enabled
> > via `global-whitespace-mode`.
> >
> > To reproduce:
>
> [...]
>
> > Then:
> >
> > 2. Run `emacs -Q`
> >
> > 3. Execute the following commands from the scratch buffer:
> >
> > (setq whitespace-style '(face lines-tail))
> > (global-whitespace-mode)
> >
> > 4. Open the created file using `M-x find-file test-file.txt`
> >
> > whitespace-mode will highlight long lines starting at 80 characters
> > instead of 100, which is what the file-local variable says.
>
> (This bug report unfortunately got no response at the time.)
>
> I'm unable to reproduce this in Emacs 26.1 and Emacs 28.  Are you still
> seeing this problem in more recent Emacs versions?
>
> --
> (domestic pets only, the antidote for overdose, milk.)
>    bloggy blog: http://lars.ingebrigtsen.no
>