there seems to be some logic which only actually (re-)draws labels (probably other stuff too) onto the screen when its content has changed from last time. My guess would be that this is done to prevent flickering.

So in your case after setting the screen black only the labels which have different content from before are actually redrawn. The only way I found to 'fix' this is that after setting the screen background to also set all the labels to let's say an empty string, e.g. "". This will force a redraw.

Thanks
Felix