Many thanks to Nicolas and Bruce for their mails and their help.
However, it appears Nicolas' MWE unfortunately does not  reproduce the
issue. It's not the footnote that is omitted/ not exported, it's the
citation in the footnote that is left out (when the footnote follows two or
more citations, of which one has a suffix). In Nicolas' MWE, there is no
citation in the footnote. What seems to me (being no export on LaTeX)
strange about Bruce's LaTeX output is that there is no command
\footnote{...} or the like. So is the footnote-part of the LaTeX output
real LaTeX? (Sorry for this naïve question.)

I've tested again.
With verbose I get the same erroneous result: the citation in the footnote
is not rendered (using the development version of org-mode, see below):
INPUT:

#+LANGUAGE: en
#+latex_header:\usepackage{minted}
#+bibliography_style: plain
#+cite_export: biblatex verbose
# Bibliography (bib file) specified within org-ref-cite in the init file

Body text [cite: @ravlic2021; @saric2010 with a SUFFIX][fn:1]
\printbibliography

#+begin_src emacs-lisp
(org-version)
#+end_src

#+RESULTS:
: 9.5

#+begin_src emacs-lisp
(emacs-version)
#+end_src

#+RESULTS:
: GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo
version 1.16.0)
:  of 2021-06-04

* Footnotes

[fn:1]Test [cite: @senker1987]

LaTeX OUTPUT (shortened):

Body text \autocites{ravlic2021}[][with a SUFFIX]{saric2010}\footnote{Test}


\printbibliography


As a work around I insert a zero width space before footnotes in the
context described.


Am Mo., 4. Okt. 2021 um 16:26 Uhr schrieb Bruce D'Arcus <bdarcus@gmail.com>:

> On Mon, Oct 4, 2021 at 10:11 AM Bruce D'Arcus <bdarcus@gmail.com> wrote:
> >
> > On Sun, Oct 3, 2021 at 6:23 PM Nicolas Goaziou <mail@nicolasgoaziou.fr>
> wrote:
> > >
> > > Hello,
> > >
> > > Elias Bounatirou <elias.bounatirou@gmail.com> writes:
> > >
> > > > Just to clarify the BUG:
> > > > Citations in a footnote in the following environment are not
> exported to
> > > > LaTeX: when the footnote follows two or more citations, of which one
> has a
> > > > suffix, i.e. for instance in
> > > >
> > > > Body text with a citation: [cite:@low2001; @mcneill2011 with a
> > > > suffix][fn:3].
> > >
> > > I cannot reproduce the problem. With the following document:
> > >
> > > --8<---------------cut here---------------start------------->8---
> > > #+bibliography: foo.bibtex
> > > #+cite_export: biblatex authoryear
> >
> > Can you try with a note-based style like verbose?
>
> Now I'm not sure.
>
> Input:
>
> ----
> #+language: en
> #+bibliography: test.bib
> #+cite_export: biblatex verbose
>
> Body text with a citation: [cite:@low2001; @mcneill2011; with a
> suffix][fn:1].
>
> #+print_bibliography:
>
> * Footnotes
>
> [fn:1] Third commentary, and then a citation: [cite:@low2001].
> ---
>
> LaTeX output:
>
> ---
> Body text with a citation: \autocites(with a
> suffix){low2001}[][]{mcneill2011}\relax[1].
>
> \printbibliography
>
>
>
> Footnotes
> ─────────
>
> [1] Third commentary, and then a citation: \autocite{low2001}.
> ---
>
> But the PDF leaves off the last citation, in the footnote.
>
> Bruce
>