Hello, CdeV. Are you using ZBrush 4R6? By some reason, the way Slice Curve works in this version has changed, I wonder if it has something to do with the new Groups capabilities in DynaMesh. Anyway, if you perform the same operations that would work in ZB4R5, you get just what you are showing… The sliced mesh doesn’t separate.
The workaround is kinda strange, but very simple. You just have to activate Colorize in the tool you are trying to slice. I am figuring out why, but it works.
If you use Auto Groups in Polygroups after the first unmask it also works, but the produced edge is usually sawtooth shaped. It makes me think that the slice curve creates an extra polygroup in the surface of separation (where the cut is produced), so that when you try to move one of the portions, that polygroup remains in its place and you can’t entirely separate the mesh.
Hope this works for you!