With apxproof, natbib citations disappear in the appendix

Question

I have just discovered the apxproof package, and it is so useful, I cannot understand how I could write conference papers without it. But there is a problem: when I move citations to the appendix, they disappear. Here is an MWE file test.tex:

\documentclass{article} 
\usepackage{natbib}
\bibliographystyle{apalike}

\usepackage{apxproof}

\begin{document}

Citing \cite{Johnson1973} in the main paper.

\begin{toappendix}
Citing \cite{Johnson1973} in the appendix.
\end{toappendix}

\bibliography{test}
\end{document}

Here is test.bib:

@phdthesis{Johnson1973,
  author       = {D. S. Johnson},
  title        = {Near-optimal bin packing algorithms},
  school       = {Massachusetts Institute of Technology},
  year         = {1973},
  type         = {PhD thesis},
}

And here is the outcome in Overleaf:

As you can see, the citation text in the appendix disappears.

Answer 1

There seems to be a issue with the combination of natbib and the usage of package bibunits by apxproof. It works, when using, for example:

\begin{filecontents*}[force]{test.bib}
@phdthesis{Johnson1973,
  author       = {D. S. Johnson},
  title        = {Near-optimal bin packing algorithms},
  school       = {Massachusetts Institute of Technology},
  year         = {1973},
  type         = {PhD thesis},
}
\end{filecontents*}

\documentclass{article} 
\usepackage{natbib}
\bibliographystyle{apalike}

\usepackage[bibliography=common]{apxproof}

\begin{document}

Citing \cite{Johnson1973} in the main paper.

\begin{toappendix}
Citing \cite{Johnson1973} in the appendix.
\end{toappendix}

\bibliography{test}
\end{document}

with common instead of a separate references:

It also works for split references without natbib or package apalike instead of natbib:

\begin{filecontents*}[force]{test.bib}
@phdthesis{Johnson1973,
  author       = {D. S. Johnson},
  title        = {Near-optimal bin packing algorithms},
  school       = {Massachusetts Institute of Technology},
  year         = {1973},
  type         = {PhD thesis},
}
\end{filecontents*}

\documentclass{article} 
\usepackage{apalike}
\bibliographystyle{apalike}

\usepackage{apxproof}

\begin{document}

Citing \cite{Johnson1973} in the main paper.

\begin{toappendix}
Citing \cite{Johnson1973} in the appendix.
\end{toappendix}

\bibliography{test}
\end{document}

But not, when changing the bibliography style of the appendix also to apalike:

\begin{filecontents*}[force]{test.bib}
@phdthesis{Johnson1973,
  author       = {D. S. Johnson},
  title        = {Near-optimal bin packing algorithms},
  school       = {Massachusetts Institute of Technology},
  year         = {1973},
  type         = {PhD thesis},
}
\end{filecontents*}

\documentclass{article} 
\usepackage{apalike}
\bibliographystyle{apalike}

\usepackage{apxproof}
\renewcommand\appendixbibliographystyle{apalike}

\begin{document}

Citing \cite{Johnson1973} in the main paper.

\begin{toappendix}
Citing \cite{Johnson1973} in the appendix.
\end{toappendix}

\bibliography{test}
\end{document}

→

./bu1.bbl

! LaTeX Error: Lonely \item--perhaps a missing list environment.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.3 \bibitem[Johnson, 1973]{Johnson1973}
                                        
? 

I would suggestion another solution: Use of biblatex and the section of segment feature of biblatex. This would also need to use option bibliography=common, but nevertheless you can generate a separate bibliography for the appendix:

\begin{filecontents*}[force]{test.bib}
@phdthesis{Johnson1973,
  author       = {D. S. Johnson},
  title        = {Near-optimal bin packing algorithms},
  school       = {Massachusetts Institute of Technology},
  year         = {1973},
  type         = {PhD thesis},
}
\end{filecontents*}

\documentclass{article} 
\usepackage[style=authoryear]{biblatex}
\addbibresource{test.bib}
\addbibresource{biblatex-examples.bib}

\usepackage[bibliography=common]{apxproof}
\renewcommand*{\appendixprelim}{\newrefsegment}

\begin{document}

Citing \cite{Johnson1973} and \autocite{knuth:ct:a} in the main paper.


\begin{toappendix}
Citing \cite{Johnson1973}  and \autocite{knuth:ct:b} in the appendix.
\end{toappendix}

\printbibliography[segment=0]

% Add this at the end, to print the segment bibliography at the end of the appendix.
\nosectionappendix
\begin{toappendix}
  \printbibliography[title=References for the
  Appendix,segment=1]
\end{toappendix}

\end{document}

or

\begin{filecontents*}[force]{test.bib}
@phdthesis{Johnson1973,
  author       = {D. S. Johnson},
  title        = {Near-optimal bin packing algorithms},
  school       = {Massachusetts Institute of Technology},
  year         = {1973},
  type         = {PhD thesis},
}
\end{filecontents*}

\documentclass{article} 
\usepackage[style=authoryear]{biblatex}
\addbibresource{test.bib}
\addbibresource{biblatex-examples.bib}

\usepackage[bibliography=common]{apxproof}
\renewcommand*{\appendixprelim}{\newrefsegment}

\begin{document}

Citing \cite{Johnson1973} and \autocite{knuth:ct:a} in the main paper.


\begin{toappendix}
Citing \cite{Johnson1973}  and \autocite{knuth:ct:b} in the appendix.
\end{toappendix}

\printbibliography[segment=0,heading=subbibliography]

% Add this at the end, to print the segment bibliography at the end of the appendix.
\nosectionappendix
\begin{toappendix}
  \printbibliography[title=References for the
  Appendix,segment=1]
\end{toappendix}

\end{document}

Both result in: