Ha, when I did that 20 minute connection at Brussels last month...got to the ICE platform in good time. No indication of if or when the train might be. Downstairs, the departure board stated (not particularly helpfully) "Train leaves from another station" (alternating between English, French and Dutch). The ticket office people made clear the "other station" concerned was Brussels North - - so we all had to get a local train to there to await the late-running ICE.
I definitely recommend using the "connections" stairway at Brussels South, rather than going all the way to the exit at the front of the platform. As well as being less busy, it's a lot more direct.