\setupbodyfont[pagella,11pt]

\defineframed
   [ACFRAME]
   [frame=off,
    offset=0pt,
    location=low,
    width=\struttotal,
    background=color,
    backgroundcolor=black,
    foregroundcolor=white,
    forgroundstyle=bold]


\defineprocessor[ACPROCESSOR][command=\ACFRAME]
\defineconversionset[ACCONVERSION][][ACPROCESSOR->n]

\defineenumeration
   [ex]
   [text=,
    distance=0.3em,
    width=fit,
    numberconversionset=ACCONVERSION,
    alternative=serried]

\starttext

\dorecurse{5}
   {\startex
    \input knuth
    \stopex}

\stoptext

Looks like a font issue -- it does not happen with LM or ebgaramond.

--
Rik