This occurred when I used an external script to set the "date" property of the component or when using the navigation buttons at the bottom of the component. It isn't the end of the world, as it's simply visual and reverts to normal when the page is reloaded, but I thought it was worth bringing up. (If there's a better place to post bug reports, I couldn't find it, but I'd be grateful if someone who knows would point me there)
As I was playing around with the component after posting this, I realized that I had made the titleBackground property partially transparent. When I switched this to completely opaque, this behavior went away. Not sure if there is some background work that the calendar component does behind the title, but apparently it's visible if the background is transparent.
In general, Java Swing (the rendering technology behind Vision) does a poor job at transparency. This is a classic symptom - a 'dirty' region that is overpainted multiple times. If there's an 'opaque' property exposed on the component, changing its value might help fix this (if you need transparency) but the simpler solution is just to not deal with transparency
