[NTG-context] Disappearing \rrbracket

[Nicola]

I have a problem with the symbol in the subject. This is a
minimal document:

\starttext
$\llbracket x\rrbracket$
$\bigl\llbracket x\bigr\rrbracket$
$\Bigl\llbracket x\Bigr\rrbracket$
$\biggl\llbracket x\biggr\rrbracket$
$\Biggl\llbracket x\Biggr\rrbracket$
\stoptext

Using ConTeXt from TeX Live 2015, \rrbracket is rendered only
in the first case (current beta is ok). Can you reproduce
this? Is there a workaround?

Nicola