Here's the fix:
After placing the slab edge, choose Unjoin from the Modify tab on the Ribbon and unjoin the Slab edge from the slab.
Next, choose Join from the same ribbon tab and be sure to choose the slab edge first, then the slab you want to join it to.
Tada! It works.
Sadly this workflow won't allow you to wrap the zones around the thickened slab edge. Maybe a fix for Revit 2013/14?
Rules for this to work:
- The material of the concrete slab must be exactly the same as the material set in the slab edge type.
- The slab edge insertion point must be the top of the slab or offset vertically in the parameters to sit at either the underside of the concrete zone i.e. not the underside of the floor construction.