This works for me, is probably not what you want, but at leat it's consistent (it uses the interline of the main text).

\definestartstop[passus][

before={\startnarrower},

after={\stopnarrower},

style={\tfxx},

]

\starttext

\subject{It looks OK here:}

\input knuth

\startnarrower

\tfxx \input knuth

\stopnarrower

\subject{It does also here:}

\input knuth

\startpassus

\input knuth

\stoppassus

\stoptext