BTW: In case that you want to split Meshes with a Mesh (from a Box [either user provided or made internally as union Box]) see attached. Using solely code since it performs a variety of checks (and reports detailed results as well).
Mesh_BooleanSplit_V1.gh (141.2 KB)
NOTE: In your data … Mesh N7 (after disjoining, checking etc etc) appears problematic. Either a Mesh Split Rhino issue OR there’s something in that thing that Valid, Manifold, Doublicate Faces checks etc etc can’t deal with.