I tried the following layout
\documentclass{article}
\usepackage[explicit]{titlesec}
\titleformat{\section}
{\vspace{1cm}\titlerule\vspace{.8ex}%
\bfseries\large}
{\Roman{section}\,|}
{.5em}
{#1}
\titlespacing{\section}{0.5ex}{*1}{*0}
\begin{document}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\section{Test}
\end{document}
But all the titles are put on the page 2, with a strong vertical overfull box: see the image below.
What is the issue and how to fix it?

\section{}. But, indeed, it works with\mbox{}although it adds some vertical space. – Colas Aug 14 '18 at 13:54