I think it is behaving correctly. If you look closely at the plot (make it bigger to see better), the revolute.w variable actually gets damped. The oscillations that are increasing in magnitude are the bounds when varying the parameter.
To see this even more clearly, you can look at a smaller part of the plot:
WSMPlot[sim3, {{"oscillant.revolute.w", "oscillant.bodyBox.density", 0.2}}, {40, 42}, PerformanceGoal -> "Speed", PlotPoints -> 200]