How to preserve spacing before + operator in `multlined` environment?

Question

How to keep the space before + operator?

Here there is the code to generate the example:

\documentclass[a4paper]{article}

\usepackage{mathtools}

\begin{document}

\begin{equation}
  \begin{multlined}
    a = b + c
  \end{multlined}
\end{equation}

\begin{equation}
  \begin{multlined}
    a = b + \\ + c
  \end{multlined}
\end{equation}

\end{document}