\colwidths{{10pt}{20pt}{30pt}}
\cals@last@row@height=1000pt %
%
\brow
\nullcell{tl}
\nullcell{t}
\nullcell{tr}
%\erow
\cals@last@row@height=40pt %
\showbox\cals@current@row
\show\cals@spanq@heights

\brow
\nullcell{l}
\nullcell{}
\nullcell{r}
%\erow
\cals@last@row@height=50pt %
\showbox\cals@current@row
\show\cals@spanq@heights


\brow
\nullcell{lb}
\nullcell{b}
\nullcell{br}
%\erow
\showbox\cals@current@row
\show\cals@spanq@heights
\showthe\cals@span@width
\showthe\cals@span@height

%
% Bug was here
%
\brow
\nullcell{ltb}
\nullcell{tb}
\nullcell{btr}
\showthe\cals@span@width