multlined breaks \substack?

Question

I have some long equations to deal with that don't fit into one line, and love the multlined-environment from mathtools to fit them into (e.g.) an align-environment, particularly if I want to package 2-3 manipulations of the quantity requiring two lines into one block (in the MWE there's only one for brevity).

However, multlined breaks down when there's a \substack inside, see MWE below. It seems that the environment somehow tries to "multline" the \substack, but I haven't managed to prevent multline from "seeing" the \substack to avoid this.

MWE (using knowledge from https://tex.stackexchange.com/a/180549/42225 and the documentation):

\documentclass[12pt,fleqn]{article}
\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}
\noindent A long equation using \texttt{multline} and \texttt{$\backslash$substack},
%
\begin{multline*}
    \mathcal{X}=\sum_{\substack{i,j,k,\ell,m,n,p,q,r,s,t\\ i+j\equiv 1 \bmod{2}}}  \binom{...}{i} \binom{...}{j} \binom{...}{k} \binom{...}{\ell} \binom{...}{m} \binom{...}{n} \cdot\ldots\\
    \ldots \cdot \binom{...}{p} \binom{...}{q} \binom{...}{r} \binom{...}{s} \binom{...}{t},
\end{multline*}
%
breaks when using \texttt{multlined} instead of \texttt{multline},
%
\begin{align*}
\MoveEqLeft[2]% default is 2em
    \mathcal{X}\\
    &\!\begin{multlined}[t][\linewidth-\mathindent-2em-\multlinegap]
        =\sum_{\substack{i,j,k,\ell,m,n,p,q,r,s,t\\ i+j\equiv 1 \bmod{2}}}  \binom{...}{i} \binom{...}{j} \binom{...}{k} \binom{...}{\ell} \binom{...}{m} \binom{...}{n} \cdot\ldots\\
        \ldots \cdot \binom{...}{p} \binom{...}{q} \binom{...}{r} \binom{...}{s} \binom{...}{t},
    \end{multlined}\\
    &=1,
\end{align*}
%
but still works fine without \texttt{$\backslash$substack},
%
\begin{align*}
\MoveEqLeft[2]% default is 2em
    \mathcal{X}\\
    &\!\begin{multlined}[t][\linewidth-\mathindent-2em-\multlinegap]
        =\sum_{i,j,k,\ell,m,n,p,q,r,s,t}  \binom{...}{i} \binom{...}{j} \binom{...}{k} \binom{...}{\ell} \binom{...}{m} \binom{...}{n} \cdot\ldots\\
        \ldots \cdot \binom{...}{p} \binom{...}{q} \binom{...}{r} \binom{...}{s} \binom{...}{t}.
    \end{multlined}\\
    &=1,
\end{align*}

\end{document}

Answer 1

With help of stackengine you can obtain:

In code below I borrow solution provided by author of stacengine in his answer in A new line inside subscript:

\documentclass[12pt,fleqn]{article}
\usepackage{mathtools}
\usepackage{stackengine}

    \begin{document}
\noindent A long equation using \texttt{multline} and \texttt{$\backslash$substack},
%
\begin{multline*}
    \mathcal{X}=\sum_{\substack{i,j,k,\ell,m,n,p,q,r,s,t\\ i+j\equiv 1 \bmod{2}}}  \binom{...}{i} \binom{...}{j} \binom{...}{k} \binom{...}{\ell} \binom{...}{m} \binom{...}{n} \cdot\ldots\\
    \binom{...}{p} \binom{...}{q} \binom{...}{r} \binom{...}{s} \binom{...}{t},
\end{multline*}
%
breaks when using \texttt{multlined} instead of \texttt{multline},
%
\begin{align*}\def\useanchorwidth{T}\stackMath
%\MoveEqLeft[2]% default is 2em
\mathcal{X}
    &\begin{multlined}[t][\linewidth-\mathindent-2em-\multlinegap]
        =\mathop{\stackunder{\sum}{\def\stackalignment{c}%
               \stackunder[2pt]{\scriptscriptstyle i,j,k,\ell,m,n,p,q,r,s,t}
                               {\scriptscriptstyle i+j\equiv 1 \bmod{2}}
               }}  
        \binom{...}{i} \binom{...}{j} \binom{...}{k} \binom{...}{\ell} \binom{...}{m} \binom{...}{n} \cdot\ldots\\
        \ldots \cdot \binom{...}{p} \binom{...}{q} \binom{...}{r} \binom{...}{s} \binom{...}{t},
    \end{multlined}\\
    &=1,
\end{align*}
%
but works fine without \texttt{$\backslash$substack},
%
\begin{align*}
\MoveEqLeft[2]% default is 2em
    \mathcal{X}\\
    &\!\begin{multlined}[t][\linewidth-\mathindent-2em-\multlinegap]
        =\sum_{i,j,k,\ell,m,n,p,q,r,s,t}  \binom{...}{i} \binom{...}{j} \binom{...}{k} \binom{...}{\ell} \binom{...}{m} \binom{...}{n} \cdot\ldots\\
        \ldots \cdot \binom{...}{p} \binom{...}{q} \binom{...}{r} \binom{...}{s} \binom{...}{t}.
    \end{multlined}\\
    &=1,
\end{align*}
    \end{document}

It seems that align, substack and multiline doeasn't low each other very much :-(, i.e. here is present a bug.