In a much simpler case, it seems to work:

\setuphead[section]
[style=\ss\tfa, after={\hrule\blank[samepage,big]}]
%~ [style=\ss\tfa, after={\hrule\blank[big]}]

\setuphead[subsection]
[style=\ss, after={\hrule\blank[samepage,medium]}]
%~ [style=\ss, after={\hrule\blank[medium]}]

\starttext
\dorecurse{10}{\section{Section}
\input knuth
\subsection{Subsection}
\input zapf}
\stoptext

Do you really need headings to be inside framed texts?