R devel - Oct 2021 - Bug (?) in vignette handling

This StackOverflow post:  https://stackoverflow.com/q/69756236/2554330 
points out that objects created in one vignette are available in a later 
vignette.  I don't think this should be happening:  vignettes should be 
self-contained.

The current answer there, https://stackoverflow.com/a/69758025/2554330, 
suggests that "R CMD check" will detect this.  However, sometimes one 
vignette can replace a standard function with a custom version, and then 
both will work without generating an error, but the second vignette 
won't do the same thing if run independently.

For example, try these pure Sweave vignettes:

-------------------------
aaa3.Rnw:
-------------------------
\documentclass{article}
%\VignetteIndexEntry{Sweave aaa3}
\begin{document}

<<>>mean <- function(x) "I am the Sweave mean"
@

\end{document}

------------------------
aaa4.Rnw:
------------------------

\documentclass{article}
%\VignetteIndexEntry{Sweave aaa4}
\begin{document}

<<>>mean(1:5)
@

\end{document}

Put these in a package, build and install the package, and you'll see 
that the mean() function in aaa4.Rnw prints the result from the 
redefined mean in aaa3.Rnw.

>>>>> Duncan Murdoch 
>>>>>     on Thu, 28 Oct 2021 13:18:54 -0400 writes:
    > This StackOverflow post:  https://stackoverflow.com/q/69756236/2554330 
    > points out that objects created in one vignette are available in a
later
    > vignette.  I don't think this should be happening:  vignettes
should be
    > self-contained.

I strongly agree.

    > The current answer there, https://stackoverflow.com/a/69758025/2554330,
    > suggests that "R CMD check" will detect this.  However,
sometimes one
    > vignette can replace a standard function with a custom version, and
then
    > both will work without generating an error, but the second vignette 
    > won't do the same thing if run independently.

    > For example, try these pure Sweave vignettes:

    > -------------------------
    > aaa3.Rnw:
    > -------------------------
    > \documentclass{article}
    > %\VignetteIndexEntry{Sweave aaa3}
    > \begin{document}

    > <<>>    > mean <- function(x) "I am the Sweave
mean"
    > @

    > \end{document}

    > ------------------------
    > aaa4.Rnw:
    > ------------------------

    > \documentclass{article}
    > %\VignetteIndexEntry{Sweave aaa4}
    > \begin{document}

    > <<>>    > mean(1:5)
    > @

    > \end{document}

    > Put these in a package, build and install the package, and you'll
see
    > that the mean() function in aaa4.Rnw prints the result from the 
    > redefined mean in aaa3.Rnw.

Is it because R is *not* run with  --no-save --no-restore
accidentally?
Without looking, I would not expect that the vignettes are run
inside the same running R (even though that may speedup things)