MWE:

\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


Maggyero