[NTG-context] A bit weird \underbar behaviour (MkIV only)

The code \usemodule[units] \starttext $y = \underbar{20 \Mega\Bit\Per\Second}$. \stoptext produces space before y and it and y get underbared. MkII produces an expected result. On the other hand, in \starttext $y = \underbar{g a w}$. \stoptext underbar doesn't appear (other letter combinations I tried seem to work). MkII can't compile this and ends with ! Incompatible list can't be unboxed. -- Vedran Miletić