I'm not sure if this is an expected behavior or a known issue, but anyway, I've found that the inner \startitemize ignores margin= and before= and probably some other options as well:

\showframe
\starttext
\startitemize[margin=2em]
\item has margin
\stopitemize
\startitemize[before=randomstuff]
\item has text before
\stopitemize

\startitemize[margin=2em]
\item .
\startitemize[margin=2em]
\item no margin
\stopitemize
\startitemize[before=randomstuff]
\item no text before
\stopitemize
\stopitemize
\stoptext

Besides, do we have an official issue tracker?

Thanks!

Best,

Sylvain