I don't know if this is a known issue, I made table with {longtable} which print out perfectly, however when viewing in TexStudio or AdobeReader the border can be missing at some zoom level. The vertical border tends to be affected most.

Most people don't print nowadays, they read on a computer so the missing borders is like I don't pay attention to detail.

This is an example, the border around the colour boxes are missing:

\begin{longtable}{|>{\raggedright\arraybackslash}p{0.42\textwidth}|>{\raggedright\arraybackslash}p{0.11\textwidth}|>{\raggedright\arraybackslash}p{0.11\textwidth}|>{\raggedright\arraybackslash}p{0.11\textwidth}|>{\raggedright\arraybackslash}p{0.11\textwidth}|}
\caption{Vulnerability Summary}\\
\hline
\textbf{Tested Infrastructure} & \cellcolor[HTML]{32CB00}\textbf{Low} & \cellcolor[HTML]{F8FF00}\textbf{Moderate} & \cellcolor[HTML]{FFC702}\textbf{Important} & \cellcolor[HTML]{FE0000}\textbf{Critical} \\