As in the Answer Hub post - if you make the spline mesh an Actor that is kept alive by the Child Actor Component - the collision will work. It’s not actually a problem with collision per se, it’s a feature of the Movement Component. It only accounts for the root of the actor it’s moving - the capsule. It will ignore other bits that are attached to the root and the spline was attached. There’s probably a hackaround for this, like changing the owner and such.
But if you ensure the spline is not a part of the moving character, the collision will / should work.
If you tried to add Instanced Static Meshes you’re using now the way you tried it with the Spline, the character wouldn’t collide against them. Same rule applies.