Why does MESH trim and MESH split fail

Hi I always have problems with the mesh in Rhino
I used the same method in another program, it works fine, but I’m curious, why not fall into Rhino.
trim-stl.3dm (18.8 MB)

Hm - probably because the mesh being cut is disjoint.
That nay not be it either - we had a similar (though not disjoint) failure come in last week, that the developer is looking into, it might be the same problem. I’ll get this on the pile, thanks.
RH-65513 MeshTrim messy result

Mesh split did work here but not with a mesh but with a extrusion (polysrf should work too).
Repair cutting mesh of your file (will remove 4 zero-length edges)
Subselect top border of the cutting mesh
Copy in place, sellast, join curves to a closed polyline
Move the extrusion down to the disjoint mesh and run splitmesh
As the resulting mesh is still connected run SplitDisjointMesh
Select by boundary the inner meshes (important: Filter must allow only selecting curves and meshes)
4 or 5 of the split meshes were not recognized correctly, maybee because of features like this in your cutting mesh: