Mr Hagen, is it the expected behaviour?

\starttext

The delimiters are correctly displayed:

\startformula

    \left\lfloor \frac{x}{y}\right\rfloor

\stopformula

The delimiters vanish:

\startformula

    \Bigl\lfloor \frac{x}{y}\Bigr\rfloor

\stopformula

\stoptext