This seems to be a bug in the control. I have logged it as such in our PITS where you can vote for it and track its status. I have updated you Telerik points as a sign of gratitude for bringing this issue to our attention.
For now I can suggest a simple work around - find the visual elements that represent the Bollinger indicator and manually set the proper StrokeThickness. I have attached a simple project demonstrating this work-around.