Looks like a bug; adding blank rows works around the problem (but
looks terrible):
\definemathmatrix[pmatrix] [left={\left(\,},right={\,\right)}]
\starttext
Digueu quines de les matrius següents són regulars. Quan ho siguin,
trobeu la inversa
\startitemize[a,columns,four]
\item \startpmatrix[n=4]
\NC \NR
\NC 2 \NC 3 \NR
\NC 6 \NC 7 \NR
\NC \NR
\stoppmatrix
\item \startpmatrix[n=4]
\NC 3 \NC 2 \NC 1 \NR
\NC 4 \NC 5 \NC 3 \NR
\NC 5 \NC 0 \NC -2 \NR
\NC
\stoppmatrix
\item \startpmatrix[n=4]
\NC 5 \NC 4 \NC 2 \NR
\NC 2 \NC 2 \NC 3 \NR
\NC 7 \NC 6 \NC 5 \NR
\NC
\stoppmatrix
\item \startpmatrix[n=4]
\NC a \NC 0 \NC 0 \NC 0 \NR
\NC 0 \NC b \NC 0 \NC 0 \NR
\NC 0 \NC 0 \NC c \NC 0 \NR
\NC 0 \NC 0 \NC 0 \NC d \NR
\stoppmatrix
\stopitemize
\stoptext
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