in the attached zsphere model, i moved 2 spheres too close to each other, and the mesh preview suddenly switched to the old classic adaptative skinning type.
I had to move spheres around to get switch to the new adaptative skinning back.
Maybe it’s a minor bug ?
This is the way it designed to work.
Please note that in your ZSpheres armature, some of the linked ZSpheres are ‘dotted’. This indicates that these ZSpheres chain overlapping other Zspheres chains and they need to be moved or scaled in order to avoid the proximity conflict. If you do not adjust the armature, then ZBrush will use classic skinning.
Note: this only applied to Adaptive skinning, unified skinning is far more forgiving:)
To also add to support, this is not a bug. If your ZSphere creation can calculate using the new ZSphere 2 scheme then it will default to the classic. This is very rare and support is correct with moving some of the zspheres around will then have it use the new scheme. It does not have to be large movement either. 
Paul