slkeranen said that they "refresh or blink." This, to me at least, isn't the same thing as disappearing entirely.
A certain amount of blinking is to be expected when using S&T. You don't even have to be navigating to notice it. Just drag the map around when zoomed in close and you'll see the same effect.
The core graphics engine in Streets & Trips is well over ten years old now, and is long overdue for some fundamental improvements.