This appears to be intentional behavior of project's caching implemented
via (vc-file-getprop dir 'project-vc) and
(vc-file-setprop dir 'project-vc project) in project-try-vc. There is no
facility, public API or private, to clear the cache en-masse. One could
reset the cache via clearing the vector vc-file-prop-obarray
(setq vc-file-prop-obarray (make-vector 17 0)) in the absence of an API.
You can observe what's in your vc-file-prop-obarray for yourself before
taking this action.

Hope that helps,

-Stephane

On Sun, Aug 4, 2024 at 4:16 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Thu, 25 Jul 2024 21:54:24 +0200
> > From:  Federico Tedin via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> >
> > In Emacs master e56e4b345a2, `emacs -q`:
> >
> > I'm having problems trying to make project.el detect a new project that
> > is contained in the directory of another project.
> >
> > I have a directory called 'scratch' which contains a '.git' directory, a
> > file 'test.py' and a directory 'foo'. The 'foo' directory contains a
> > file called 'foo.py'.
> >
> >
> > ~/scratch/
> >     .git/
> >     main.py
> >     foo/
> >         foo.py
> >
> >
> > If I open 'main.py', `(project-current)' evals to the expected: `(vc Git
> > "~/scratch")'.
> >
> > If I open 'foo.py', `(project-current)' also evals to `(vc Git
> > "~/scratch")', which is expected.
> >
> > However if now I cd into 'foo/' and run `git init`, then I would expect
> > project.el to now consider 'foo.py' to be in another project - `(vc Git
> > "~/scratch/foo")'. However, if I evaluate `(project-current)' when
> > visiting 'foo.py', I still get `(vc Git "~/scratch")'.
> >
> > If I kill the buffer visiting 'foo.py' and open the file again, I get
> > the same result.
> >
> > Interestingly, if I run 'M-x project-remember-projects-under' with
> > '~/scratch/foo' as path, it does inform me that the new project has been
> > found. However visiting 'foo.py` still results in `(vc Git "~/scratch")'
> > as the current project.
> >
> > If I restart Emacs then the problem is solved; 'foo.py' is correctly
> > filed under project `(vc Git "~/scratch/foo")'.
> >
> > The fact that this works correctly after restarting makes me think
> > that there must be some runtime state set up that is preventing the
> > desired behaviour to happen.
>
> Dmitry, any comments or suggestions?
>
>
>
>