[Tex/LaTex] Missing thick vertical line when using multicolumn

Question

For some reason couple of the thick vertical lines are missing from the first row (still there is the thin line):

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{multirow}
\usepackage{bigstrut}
\usepackage{adjustbox}
\usepackage{makecell}
\usepackage{amssymb}
\usepackage{array}
\newcolumntype{?}{!{\vrule width 2pt}}
\newcolumntype{M}[1]{>{\raggedleft\arraybackslash}m{#1}}

\begin{document}
\renewcommand{\arraystretch}{0.66}
\begin{table}[htbp]
  \centering
  \caption{Review of methods and ...}
  \begin{adjustbox}{center}
  \setlength\tabcolsep{2pt}
    \begin{tabular}{|c?c|c?c|c|c|c|c?c|c|c|}
    \hline
       & \multicolumn{2}{c|}{\linespread{1}\selectfont{}\centering Bugs' info} & \multicolumn{5}{c|}{Expertise info}         & \multicolumn{3}{c|}{Based on} \\
    \hline
    Method & \parbox{1.7cm}{\linespread{1}\selectfont{}\centering Title + description} & \parbox{0.8cm}{\linespread{1}\selectfont{}\centering Meta} & \parbox{1.9cm}{\linespread{0}\selectfont{}\centering Bug fixing; title / description} & \parbox{1.6cm}{\linespread{1}\selectfont{}\centering Being a committer} & \parbox{1.1cm}{\linespread{1}\selectfont{}\centering Tossing history} & \parbox{0.8cm}{\linespread{1}\selectfont{}\centering Meta} & \parbox{1.3cm}{\linespread{1}\selectfont{}\centering Changed code} & \parbox{1.6cm}{\linespread{1}\selectfont{}\centering Final committer} & \parbox{1.3cm}{\linespread{1}\selectfont{}\centering Closer / resolver} & \parbox{1.3cm}{\linespread{1}\selectfont{}\centering Drafted assignee} \\
    \hline
    AA & \checkmark & & \checkmark & & & & & \checkmark & \checkmark & \\\hline
    BB & \checkmark & & \checkmark & & & & & \checkmark & \checkmark & \\\hline
    CC & \checkmark & \checkmark & \checkmark & & \checkmark & \checkmark &  & \checkmark & \checkmark & \\\hline
    \end{tabular}%
    \end{adjustbox}
  \label{tab:previousMethods}%
\end{table}%
\end{document}

Answer 1

I propose this other layout, with rows and columns switched, and less horizontal and vertical rules. The caption package is loaded to ensure a decent vertical spacing between table and caption above:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{multirow}
\usepackage{bigstrut}
\usepackage{makecell, booktabs, caption}
\usepackage{amssymb}
\usepackage{array}
\usepackage[table, svgnames]{xcolor}
\newcolumntype{M}[1]{>{\raggedleft\arraybackslash}m{#1}}
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.3pt}

\begin{document}

\begin{table}[htbp]
  \centering
  \renewcommand{\arraystretch}{0.66}
  \caption{Review of methods and ...}
  \setcellgapes{3pt}\makegapedcells
  \arrayrulecolor{Gainsboro}
  \begin{tabular}{!{\color{Gainsboro}\vrule width2pt}llccc!{\color{Gainsboro}\vrule width2pt}}
    \specialrule{2pt}{0pt}{0pt}
    \multicolumn{2}{!{\color{Gainsboro}\vrule width2pt}c}{Method} & AA & BB & CC \\
    \specialrule{0.6pt}{0pt}{0pt}
    \multicolumn{5}{!{\color{Gainsboro}\vrule width2pt}l !{\color{Gainsboro}\vrule width2pt}}{\bigstrut\em Bugs info} \\
      & Title +description & \checkmark & \checkmark & \checkmark \\
      & Meta & & & \checkmark \\
    \specialrule{0.6pt}{0pt}{0pt}
    \multicolumn{5}{!{\color{Gainsboro}\vrule width2pt}l !{\color{Gainsboro}\vrule width2pt}}{\em Expertise info}\\
    & \makecell[l]{Bug fixing;\\ title/description} & \checkmark & \checkmark & \checkmark \\
      & Being a committer & & & \\
      & Tossing history & & & \checkmark \\
      & Meta & & & \checkmark \\
      & Changed code & & & \\
    \specialrule{0.6pt}{0pt}{0pt}
    \multicolumn{5}{!{\color{Gainsboro}\vrule width2pt}l !{\color{Gainsboro}\vrule width2pt}}{\bigstrut\em Based on} \\
      & Final committer & \checkmark & \checkmark & \checkmark \\
      & Closer/resolver & \checkmark & \checkmark & \checkmark \\
      & Drafted assignee & & & \\
    \specialrule{2pt}{0pt}{0pt}
  \end{tabular}%
  \label{tab:previousMethods1}%
\end{table}

\end{document}