Superfluous space at start of aligned inside multlined

[7Z0t99]

In the following LaTeX document, which uses the aligned environment inside multlined, there is an superfluous space after the opening { compared to when multlined is replaced by split:

\documentclass[a4paper]{article}
\usepackage{mathtools}
\begin{document}
\begin{gather}
\begin{multlined}
%\begin{split}
\begin{rcases}a_1\\
a_2\end{rcases} = \\
\left\{\begin{aligned}b_1\\
b_2\end{aligned}\right\}x
%\end{split}
\end{multlined}
\end{gather}
\end{document}

Example with multlined: Example with split:

@davidcarlisle has proposed to set the aligned environment outside the multlined environment as a workaround, see https://tex.stackexchange.com/questions/720874/superfluous-space-at-start-of-aligned-inside-multlined

I would be glad if multlined could be updated such that there is no superfluous space with aligned.