Latex 超过 9 个章节的目录间距问题

Question

当章节号超过 9 时，: 和章节标题之间的间距会消失。这是我的代码（输入文件除了章节标题外都是空的，以 \endinput 和输出结尾。

为什么会发生这种情况？我该如何解决？如果我更改任何参数，似乎都没有任何效果。

\documentclass[11pt,draft]{book}
\usepackage{tocloft}
\usepackage{etoolbox}

\renewcommand{\cftpartfont}{\bfseries}
\renewcommand{\cftpartpagefont}{\bfseries}
\renewcommand{\cftpartpresnum}{PART~}
\renewcommand{\cftpartaftersnum}{:}
\renewcommand{\cftpartnumwidth}{6em}

\renewcommand{\cftchapfont}{\bfseries}
\renewcommand{\cftchappagefont}{\bfseries}
\renewcommand{\cftchappresnum}{Chapter~}
\renewcommand{\cftchapaftersnum}{:}
\renewcommand{\cftchapnumwidth}{6em}




\renewcommand{\partname}{PART}

\newcommand{\originalappendix}{}
\let\originalappendix\appendix
\renewcommand{\appendix}{%
  \originalappendix
  \addtocontents{toc}{%
    \protect\renewcommand{\protect\cftpartpresnum}{}%
    \protect\renewcommand{\protect\cftchappresnum}{Appendix~}%
    \protect\renewcommand{\protect\cftchapnumwidth}{7em}%
  }%
  \cleardoublepage
  \addcontentsline{toc}{part}{APPENDICES}%
  \part*{APPENDICES}%
}
\begin{document}
\frontmatter


\setcounter{tocdepth}{1}
\tableofcontents
\mainmatter
\part{}
\input{Ch1}
\input{Ch2}
\input{Ch3}
\input{Ch4}
\input{Ch5}
\input{Ch6}
\input{Ch7}
\input{Ch8}
\input{Ch9}
\input{Ch10}
\input{Ch11}
\input{Ch12}
\input{Ch13}
\input{Ch14}
\input{Ch15}
\input{Ch16}
\input{Ch17}
\input{Ch18}
\input{Ch19}
\input{Ch20}

\end{document}

Contents
PART I
1
Chapter 1: Chapter 1
3
Chapter 2: Chapter 2
5
Chapter 3: Chapter 3
7
Chapter 4: Chapter 5
9
Chapter 5: Chapter 5
11
Chapter 6: Chapter 6
13
Chapter 7: Chapter 7
15
Chapter 8: Chapter 8
17
Chapter 9: Chapter 9
19
Chapter 10:Chapter 10
21
Chapter 11:Chapter 11
23
Chapter 12:Chapter 12
25
Chapter 13:Chapter 13
27
Chapter 14:Chapter 14
29
Chapter 15:Chapter 15
31
Chapter 16:Chapter 16
33