The international monetary fund maintened its forecast 'of/ for' weak global growth??? According to me it should be 'of' as the news of weak global growth makes up the forecast and hence is a part of the forecast.
"...its forecast of weak global growth" is correct.
It has the same function in the sentence as (as you say) "news of weak growth", "predictions of weak growth", "consequences of weak growth", etc.
It's the same construction as the words you used: news of weak global growth.