While the short answer is "I don't know," the longer answer is that it depends on MainStage, which I haven't seen in a few years. Ableton, for one, sends this information back nicely, but anyway...
Have a look with Snoize MIDI Monitor and see what's going on. That's one side of it. The other side is to get on MainStage forums and see what people have to say. Please do come back here with your findings.
Someone told us recently that Mainstage is sending back parameter info, again, after taking a short break for a few years.