You get the menu id for the currect request.
The Ajax uses another request than your normal page load. That is why Joomla is using some default or cached value to determine active menu.
You should add the current ItemId as parameter to the ajax request itself instead if it is important for your modules function.
Administrator side's ContentModelArticles does not support filtering by ID.
ContentModelArticle does support it. The state property name is article.id:
If this is a frontend module you might want to use the frontend articles model:
// Register frontend model classes.