I am seeing the same thing. I select an object to split:
then I pick a cutting object
but Rhino chooses to conduct BooleanUnion instead
… and discover as a bonus that Rhino has also conducted a split but it hasn’t split the target object but the cutting object:
I am having to spend a lot of time making extra copies of things to replace objects Rhino fixes against things the instructions say it won’t, and it’s hard learning to ignore the instruction text to give instructions based on experimental discovery of how Rhino’s Boolean operations work, in contrast to the clear instructions.
Also, the toggle for deleting input objects doesn’t seem to behave as one expects, choosing to delete input objects leaves the original objects right where you started, overlapping your intended objects, potentially screwing up later efforts to count objects or calculate volumes. Because I can’t tell what Rhino will delete, and must experimentally select everything to see what’s left afterward, I’ve had to give up using the option at all and have made a discipline out of manually deleting what I don’t need to keep instead of risking Rhino throwing away what I need to keep.