Grab mechanics and default controllers vs alternate controllers

Purely speculation because Unity does not work on the Toolkit project, but from my experience with it the controller models they load (the default windows ones) are GLTF models pulled from a particular DLL. These do not come with collision shapes to detect whether you're grabbing. You would need to attach your own collider component at some point during runtime after the models themselves are loaded.

An alternate model would work because in theory it's a prefab with a collider component included in that as you apply it to the alternate model field in the Inspector panel.

But again, your mileage may vary on my response. Questions on specific functionality are better fronted over at the HoloToolkit GitHub page. https://github.com/Microsoft/MixedRealityToolkit-Unity