Estees un MWE al dorso.
Código del ejemplo:
\documentclass{book}
\usepackage{indentfirst}
\usepackage{nccmath} % align & gather doesn't use shortskip: https://tex.stackexchange.com/a/386266/206774
\useshortskip % nccmath seems to fix that even without this line
\begin{document}
1st case case case case case case case case
\[\begin{cases}
D: \text{wheel diameter} \\
A = l \sin{\beta_1} \\
\end{cases}\]
2nd case case case case case case case case
\[\begin{cases}
D: \text{wheel diameter} \\
A = l \sin{\beta_1} \\
\end{cases}\]
\end{document}
Este es el problema:
Parece que sólo el primero casesse ve afectado.