that's because now you have a hbox
anyway, i uploaded a beta with a fix but still, keep in mind that you need to avoid empty lines:
\startTEXpage[offset=0.5ex,align={lohi,middle}]
\bTABLE[frame=off,loffset=1ex,roffset=1ex,align={middle,lohi}]
\bTR
\bTD A \eTD
\bTD B \eTD
\bTD C \eTD
\eTR
\eTABLE
\blank[line]
\bTABLE[frame=off,loffset=1ex,roffset=1ex,align={middle,lohi}]
\bTR
\bTD D \eTD
\bTD E \eTD
\bTD F \eTD
\eTR
\eTABLE
\stopTEXpage
as a \noindent is injected at the beginning and that has the side effect of an empty line then forcing width of hsize