Looking at the HandsCharacterBase and hovering over RightFingerTip, you may notice that it is not a standard SphereComponent as the fist spheres are, but rather a ContinuousOverlapSphereComponent.
This is a specialized component that prevents the fingertip from moving through the button without triggering it. The current implementation of SphereComponent does not support continuous overlap detection (only continuous collision detection, which can be enabled with "Use CCD").
This is a useful blueprint node that will run from the Completed pin once per tick until Break is hit.