\continuewhenlmtxmode

\starttext
    \startMPpage[offset=10pt]
        draw lmt_outline [
            text = "\mathematics{\sqrt{2}-1}"
        ] ;

        draw lmt_outline [
            text = "\mathematics{\sqrt[rule=no]{2}-1}"
        ] shifted (0,-1cm) ;

        draw lmt_outline [
            text = "\mathematics{\sqrt[rule=yes]{2}-1}"
        ] shifted (0,-2cm) ;
    \stopMPpage
\stoptext