UCI Regulations for Track cycling provide a very loose definition for velodromes (see page 75) even up to the Olympic level. The only strict requirements beyond track width and markings are that velodromes must be 133-500m long (250m exactly for Olympic events), the track shape should consist of two parallel straights connected by two bends and that there is a smooth transition between the straights and the bends. There is no requirement for a consistent radius or banking throughout each bend. There is no requirement that the straights should be the same length or relative elevation either.
Exact dimensions of various velodromes are difficult to find, however to give one example How the velodrome found its form features an interview with the engineers responsible for the velodrome used for the London 2012 games where they note:
The track in the London Velodrome doesn't have the usual reflection symmetry you find in buildings. "If you folded the track in half lengthwise, the two halves wouldn't match," says Weir. The track does have rotational symmetry, it looks the same if you spin it by a half-turn. But the slope of the track going into and out of the turns is not the same. "This is simply because you always cycle the same way around the track, and you go shallower into the turn and steeper out of it."
While it is likely that most velodromes feature rotational symmetry and many of those will also feature reflective symmetry this is not a requirement for a UCI homologated track.