Hi,
There seems to be a problem with the structure information added when using \startmatrix:
\setuptagging[state=start]
\starttext
\math{
\startmatrix
\NC A \NR
\NC B \NR
\stopmatrix}
\stoptext
The log reports an open chain: document>1 => mtr>1 => mtd>1 => mtr>2 => mtd>2 => mtr>3
This may be a bug.
Kind regards,
Christoph