From the docs:
Getting Scene data on Groups
When getting per-component data, such as PointPosition, the results are correct only when all objects in the group have the same number of components. As a workaround, one possible way to get, for example, all point locations is to plug the Get Data (group) node’s value into the Geometry port of a Generate Sample Set node with Emission Type set to Point and Rate Type set to All Points.
Compare with: Getting point positions from a group