Hi Hans,
With the upgraded version of lmtx, the parentheses of the matrix are no longer adjusted to the content.
Thank you.
Fabrice

\definemathmatrix [pmatrix][matrix:parentheses][simplecommand=MATRIX]

\starttext

\math{\MATRIX{\MPcode{draw (0,0) -- (1cm,0);}  ;\MPcode{draw (0,0) -- (1cm,0);} }}

\stoptext