\documentclass[landscape]{article}

\pdfpagewidth=\paperwidth
\pdfpageheight=\paperheight
\parskip=\baselineskip
\textwidth=\paperwidth
\addtolength\textwidth{-4cm}%
\evensidemargin=2cm
\addtolength\evensidemargin{-1in}
\addtolength\evensidemargin{-\hoffset}
\oddsidemargin=\evensidemargin

\usepackage{datatool}

\begin{document}
\def\tempa{%
 
{InsideBracesA},OutsideBracesA,{InsideBracesB},OutsideBracesB,{InsideBracesC},OutsideBracesC%
}

All the braces are there there before the replacement:

\texttt{\meaning\tempa}%

\DTLsubstituteall{\tempa}{,}{/}

All the braces are gone after the replacement:

\texttt{\meaning\tempa}%

\null\hrulefill\null

\def\tempa{%
 
{InsideBracesA},OutsideBracesA//{InsideBracesB},OutsideBracesB//{InsideBracesC},OutsideBracesC%
}

All the braces are there there before the replacement:

\texttt{\meaning\tempa}

%Let's replace all commas:

\DTLsubstituteall{\tempa}{,}{/}

Only the first pair of braces is gone after the replacement:

\texttt{\meaning\tempa}

\end{document}
