\startuniqueMPgraphic{foo}
    fill OverlayBox withcolor lightgray;
\stopuniqueMPgraphic

\defineoverlay[foo][\uniqueMPgraphic{foo}]

\starttext

\framed[offset=overlay,framecolor=red]\bgroup
\vbox\bgroup\normaloffinterlineskip
  \setupframed[background=foo,offset=overlay,height=10mm,width=60mm,rulethickness=1pt]% no problems without mp background
  \framed{unwanted interlineskip}
  \framed{in mkiv}
  \framed{}
  \framed{}
  \framed{}
\egroup
\egroup

\stoptext