Dragging an object in the top view, when it is over but not touching another object, breaks the group.
Only in top view, only if the 2nd object is present - deleting, locking or hiding the 2nd object makes the problem go away.
The 3dm file is just these two objects, extracted from a much larger one.
There is a definite bug in the grouping in V8, or I’m going nuts…
I have a group of just two polysurfaces.
If its on its own in the scene,(ie everything else hidden), both select together as you’d expect a group to behave.
If its part of a larger scene, it will behave as two separate objects.
So now I’ve just cut and pasted the whole model into V7, where everything works as expected.
I note the ungroup command has been added to V8; maybe this has introduced this problem?
I don’t know, but for now I’m back to working in V7.
I can’t post the file here - if someone wants it let me know.