alineación incorrecta cuando se utilizan subecuaciones y alineación en combinación

Question

\documentclass[a4paper, 12pt, fleqn]{book}
\usepackage[utf8,ansinew]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{siunitx}  
\usepackage[fleqn,centertags]{mathtools}
\begin{document}

This is ordinary text and below 3 blocks of equations. The first one without using subequations.
\begin{alignat}{2}
 &a     &&=b ~,\label{1a}\\[1ex]
 &alpha &&=beta~,\label{1b} \\[1ex]
 &c     &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923}{kN/m^2} \label{1c} \\[1ex]
 &cc    &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923.08}{kN/m^2} \label{1d}
\end{alignat}%
The second block uses the subequation environment.
{\allowdisplaybreaks [4]
\begin{subequations}\label{1}
%\noeqref{1a,1b,1c,1d}
\begin{alignat}{2}
 &a     &&=b ~,\label{1a}\\[1ex]
 &alpha &&=beta~,\label{1b} \\[1ex]
 &c     &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923}{kN/m^2} \label{1c} \\[1ex]
 &cc    &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923.08}{kN/m^2} \label{1d}
\end{alignat}
\end{subequations}}%
The alignment on the left side is fine in both cases. However, in the second case there is too much horizontal space left of the "=" sign. When the equation (5d) is removed, or if I reduce the length of (5d) by deleting one or more digits again, the alignment is fine, as can be seen below.
{\allowdisplaybreaks [4]
\begin{subequations}\label{1}
%\noeqref{1a,1b,1c,1d}
\begin{alignat}{2}
 &a     &&=b ~,\label{1a}\\[1ex]
 &alpha &&=beta~,\label{1b} \\[1ex]
 &c     &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923}{kN/m^2} \label{1c} \\[1ex]
 &cc    &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923}{kN/m^2} \label{1d}
\end{alignat}
\end{subequations}}
\end{document}   

Esto es lo que obtengo:

Answer 1

Estás teniendo mala suerte. Como puede ver, las etiquetas en la segunda pantalla están desalineadas: probablemente amsmathdebería funcionar mejor en este caso y, al menos, emitir una advertencia de que la alineación es demasiado amplia.

Como estás usando fleqn, puedes resolver el problema pretendiendo que la ecuación larga es en realidad un poco más corta.

\documentclass[a4paper, 12pt, fleqn]{book}
\usepackage[T1]{fontenc}
\usepackage{siunitx}  
\usepackage[fleqn,centertags]{mathtools}
\usepackage{amsmath}

\begin{document}

This is ordinary text and below 3 blocks of equations. The first one without using subequations.
\begin{alignat}{2}
 &a     &&=b ~,\label{1a}\\[1ex]
 &alpha &&=beta~,\label{1b} \\[1ex]
 &c     &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923}{kN/m^2} \label{1c} \\[1ex]
 &cc    &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923.08}{kN/m^2} \label{1d}
\end{alignat}
The second block uses the subequation environment.
\begin{subequations}\label{1}
\begin{alignat}{2}
 &a     &&=b ~,\label{x1a}\\[1ex]
 &alpha &&=beta~,\label{x1b} \\[1ex]
 &c     &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923}{kN/m^2} \label{x1c} \\[1ex]
 &cc    &&=\frac{\num{72000}\cdot\num{50000}\,(1 - \num{0.30})}{\num{50000}\,(1 - \num{0.30}) -
           2\cdot\num{72000}\cdot\num{0.25}^2} = \SI{96923.08}{kN/m^2}
           \hspace{-2em}
           \label{x1d}
\end{alignat}
\end{subequations}

\end{document}  

1. Nunca lo use \allowdisplaybreaksen la versión final de un documento. Puede usarlo en el preámbulo al escribir el documento, pero al final los saltos de página deben elegirse cuidadosamente.

2. No tiene sentido cargar ansinew. Quitar la inputencllamada.

3. ¿Por qué la alineación izquierda de los lados izquierdos?