Yes, this is a real effect. In my hometown of Pilsen which gave the beer its name, every kid knows the physics because they're taught to drink and observe Pilsner Urquell, Gambrinus, and other top brands already when they enter the kindergarten. See e.g.
The explanation is that the bubbles in the middle of the glass go up immediately - because it's easier for them to move if they're in the middle. But by going up, they're pushing the liquid in the upward direction, too. Obviously, the liquid has to return to the bottom in some way - to the place that was initially occupied by the rising bubble.
What is the path through which the liquid is returning over there? Well, the path goes near the boundaries of the glass - near the glassy material itself - where the bubbles are easier to be seen. So the rising bubbles at the center create some circulation patterns that go in the opposite direction (down) near the glassy boundary and that's where the bubbles are very easy to be observed (they're not hiding behind other bubbles and opaque liquid). With some exaggeration, the circulation patterns may look like this:
The effect doesn't last long because the bubbles ultimately achieve a higher speed than the circulation speed (note that the bubbles moving up also accelerate, at least for a while). After some time, the liquid is returning to the bottom in between the bubbles, more or less uniformly in the whole horizontal area of the glass.
I suspect that dark beers are more likely to be biased in the direction that the "nearby bubbles, near the glass, are easier to be seen" which is why the visual impression that "the motion down prevails" should be stronger for darker beers such as Guinness.