Why is this happening?
If this is a bug, would Solidworks thank me for finding it?
I may have filmed it all too fast but what happens is that I mate parts to be concentric and they become concentric for a second and than move apart.
Yet there is a concentric mate in the mates folder of the model tree.
It is obviously not concentric and two centers of two arched surfaces are marked at different places on the 3D drawing.