Unfortunately, the following MNWE results in an error when compiled with the latest version of ConTeXt LMTX:

\setuptagging[state=start]
\definemathmatrix[mymatrix][left={\left(},right={\right)}]
\starttext
\startformula
  \startmymatrix
    \NC A \NC B \NR
  \stopmymatrix
\stopformula
\stoptext

Omitting "\setuptagging[state=start]" removes the error.

A fix would be highly appreciated!