\begin{center} 和 \end{center} 不起作用

Question

在下面显示的 MWE 中，虽然我使用了 \begin{center} 和 \end{center}，但文本无法居中。

用于突出显示文本的自定义命令来自：突出显示文本背景

\documentclass[a4paper,12pt,twoside]{book}
\usepackage[a4paper,top=2.5cm,bottom=2.5cm,margin=2.5cm,bindingoffset=0.5cm]{geometry} 
\usepackage{pdflscape}
\usepackage[hidelinks,colorlinks=true,breaklinks,allcolors=blue]{hyperref} 
\usepackage[bidi=basic,layout=lists.tabular]{babel}
\babelprovide[import=ar,mapdigits,main]{arabic}
\babelprovide[import=en,language=Default]{english}
\babelfont{rm}{Latin Modern Roman}
\babelfont[arabic]{rm}{amiri}
\babelfont[english]{rm}{Times New Roman}
\usepackage{booktabs}
\usepackage{makecell} 
\usepackage{boldline} 
\usepackage{fancyhdr} 
\usepackage[normalem]{ulem}
\usepackage[breakable,skins]{tcolorbox}

%% CUSTOM COMMAND TO HIGHLIGHT TEXT
\newtcolorbox{HL}[1][]{%
  frame empty, sharp corners, top=0pt, right=0pt, bottom=0pt, left=0pt,
  boxsep=0pt, toprule=0pt, rightrule=0pt, bottomrule=0pt, leftrule=0pt,
  colback=lightgray, fontupper=\bfseries,
  before upper=\selectlanguage{arabic}%
    {\color{white}\vrule width 1.5em},
  after upper={{\color{white}\leaders\vrule\hfill\null}},
  before=\noindent\begingroup\selectlanguage{english},after=\endgroup,
#1}




\begin{document}


\vspace{-1.50cm}
\begin{center}
\begin{bfseries}
\begin{HL}[colback=gray!25]
mmmmmm
\end{HL}
\end{bfseries}
\end{center}
\begin{center}
\begin{bfseries}
\begin{HL}[colback=gray!25]
mmmmmmm
\end{HL}
\end{bfseries}
\end{center}
\begin{center}
\begin{bfseries}
\begin{HL}[colback=gray!25]
MMMMMMMMMMM
\end{HL}
\end{bfseries}
\end{center}
\vspace{-0.5cm}
\end{document}