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