I refer to the same situation as @tommsch at Brackets with the same linewidth but in
ConTeXtinstead ofLaTeX.
In ConTeXt the two first matrices are clean but the third one has the same problem as @tommsch described even if the spacing is far better.
\definemathmatrix[bmatrix][left={\left[\,}, right={\,\right]}]
\starttext
\startformula
\startbmatrix
\startbmatrix
\NC 1 \NC 0 \NC 0 \NR
\NC 2 \NC 0 \NC 0 \NR
\NC 1 \NC 0 \NC 0 \NR
\stopbmatrix
\NR
\startbmatrix
\NC 1 \NC 1 \NC 0 \NR
\NC 1 \NC 1 \NC 0 \NR
\stopbmatrix
\NR
\startbmatrix
\NC 1 \NC 2 \NC 1 \NR
\stopbmatrix
\NR
\stopbmatrix
\stopformula
\stoptext

natural tablesorextreme tablesin combination with ametapostbackground I suggest to send your problem to the ConTeXt mailing list. Other mechanism like fractions let you create special layouts withmetapostwhich could be an option for this problem. – Wolfgang Schuster Dec 31 '18 at 14:36