What specific observation regarding the motion of W3OH complicated older distance calculations?

Answer

It orbited the galactic center slower than the general rotation and had a slight inward trajectory

The object W3OH displayed anomalous motions, orbiting the galactic center slower than the general galactic rotation and showing a slight trajectory inward toward the center, which skewed distance calculations based on simple rotational models.

What specific observation regarding the motion of W3OH complicated older distance calculations?
galaxyastronomyDistanceEarthMilky WayPerseus Arm