boxed proof compile error when there is no word in the theorem

Question

I have a trivial theorem that is like:

\begin{Prop}
\[1+1=2\]
\begin{proof}
Trivial.
\end{proof}
\end{Prop}

and my overleaf marks the "Trivial" red and says:

Something's wrong--perhaps a missing \item.

Everything works as intended, but the red mark just won't go.

Then, I tried adding some comment to the proposition

\begin{Prop}
This statement is true:
\[1+1=2\]
\begin{proof}
Trivial.
\end{proof}
\end{Prop}

and the red mark is gone.

Why is this?

This is the code to duplicate the problem:

\documentclass{article}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{mdframed}

\newtheorem{proposition}{Proposition}
\newmdtheoremenv{Prop}{Proposition}

\surroundwithmdframed{proof}

\begin{document}

\begin{Prop}
    \[1+1=2\]
    \begin{proof}
        Trivial.
    \end{proof}
\end{Prop}

\end{document}

The result:

The error:

Also, if I remove the \usepackage{amsmath}, the line would be fine. This is also strange.

Answer 1

As Ulrike told in a comment, I would recommend to use tcolorbox instead of mdframed:

\documentclass{article}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{tcolorbox}
\newtheorem{Prop}{Proposition}
\tcolorboxenvironment{Prop}{colback=white}
\tcolorboxenvironment{proof}{colback=white}
\begin{document}

\begin{Prop}
    \[1+1=2\]
    \begin{proof}
        Trivial.
    \end{proof}
\end{Prop}

\end{document}

Instead of first defining the new theorem environment using \newtheorem and afterwards converting it into a tcolorbox environment using \tcolorboxenvironment, you can alternatively directly define a boxed theorem environment using \NewTcbTheorem. Please, see the tcolorbox manual for more information.