Hi,
The first section begins, so the title appears footer on each page as the second section does not start. Then the second section starts and was the title of the second section as the third section does not start and so on.

Thanks,

Fabrice

\setuppagenumbering
  [alternative=doublesided]

\setupfootertexts[margin][][{\getmarking[chapter]}]

\starttext

\chapter{First}

\section{one}

\section{two}

\chapter{Second}

\section{one}

\section{two}

\stoptext