Setfirstline does not seem to be working well with most fonts, perhaps all. The example below shows the issue.

\define             \Fonts
                    {libertinus,ebgaramond,termes,antykwa,
                     cambria,minion,bonum,heros,pagella,iwona}
\define[1]          \Setupbodyfont{\setupbodyfont[#1,11pt]}
\define[1]          \DisplayExample{\switchtobodyfont[#1]\getbuffer}

\setupfirstline     [alternative=line,
                     style=\setfontfeature{smallcaps}]
\setupinitial       [location=text,n=2,color=darkred]

\setuphead          [chapter][
                     after={\blank[big]\setfirstline\setinitial}]
\setuphead          [section][continue=yes,
                     after=\setfirstline]

\startbuffer
    \startchapter[title=Initial and first line: \Word{\fontclass}]
        \startparagraph
            \input darwin
        \stopparagraph
     \startsection[title=First line only]
        \startparagraph
            \input montgomery
        \stopparagraph
        \startparagraph
            \input sapolsky
        \stopparagraph
    \stopsection
    \stopchapter
\stopbuffer

\expandafter\processcommalist\expandafter[\Fonts]\Setupbodyfont

\starttext

\expandafter\processcommalist\expandafter[\Fonts]\DisplayExample

\stoptext

--
Rik