Minimal code:

\definecolumnset[columntest][n=2,balance=yes]
\definecolumnsetspan[wide][n=2]
\starttext
\startcolumnset[columntest]
    \startcolumnsetspan[wide]
        \dorecurse{3}{\input knuth\par}
    \stopcolumnsetspan
    \dorecurse{10}{\input knuth\par}
\stopcolumnset
\stoptext

There is no columnsetspan, and a wide frame shows up instead.

--
Best Regards
Chen