This is an Overleaf MWE.
Code from the example:
\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}
This is the problem:
It seems that only the first cases is affected.

\shortskipin the present context is rather questionable. What are you trying to achieve? – Mico Nov 07 '20 at 07:49alignin my project is placed too far from the previous paragraph.\useshortskipfix this for me. – Minh Nghĩa Nov 07 '20 at 09:56\useshortskipcan not fix that) – David Carlisle Nov 07 '20 at 15:43\useshortskiponly applies to the next equation. So only the first one here. It is usually applied immediately before, using it in the preamble is a bit odd, if not exactly an error. – David Carlisle Nov 07 '20 at 15:49