[NTG-context] Custom sized delimiters vanish

Dear list, In TL2016 custom sized delimiters of \lvert and \rvert simply vanish.  The same happens when I replace \lvert (or \rvert) by \vert and |.  MWE is below. Cheers, Henri \starttext \startformula   \left \lvert \frac{a}{b} \right\rvert   \quad   \Biggl\lvert \frac{a}{b} \Biggr\rvert   \quad   \biggl\lvert \frac{a}{b} \biggr\rvert   \quad   \Bigl \lvert \frac{a}{b} \Bigr \rvert   \quad   \bigl \lvert \frac{a}{b} \bigr \rvert \stopformula \stoptext