[Tex/LaTex] Why doesn’t \par end this \centering

groupinghorizontal alignment

As per this answer, \par should properly end the effect of \centering. But in the following case, it doesn't. Why?

\documentclass{article}

\makeatletter
    \newcommand\@mytitle{} % create macro for title
    \newcommand\mytitle[1]{\renewcommand\@mytitle{#1}}
    \newcommand\@myauthor{} % create macro for author
    \newcommand\myauthor[1]{\renewcommand\@myauthor{#1}}
    \newcommand{\articletitle}{%
        \centering%
        \fontsize{18bp}{18bp}\selectfont%
        \@mytitle\par%
        \vspace{\baselineskip}%
        \fontsize{14bp}{14bp}\selectfont%
        \@myauthor\par%
        \fontsize{12bp}{12bp}\selectfont%
        \vspace{2\baselineskip}\par}%
\makeatother

\mytitle{This is my title}
\myauthor{This is the author}

\AtBeginDocument{\articletitle}

\begin{document}
I start writing here
\end{document}

enter image description here

Best Answer

\par does not end the effect of \centering, the end of the paragraph is just the place, where TeX uses the values set by \centering. A group can be used to limit the effect of \centering. After the group, the settings before the group are in force again.

\begingroup
  \centering
  ...
  \par
\endgroup

or {\centering ...\par}.

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