Below animation explains it better than words, but sometimes I find that I can’t move a line or corner off of a specific plane without first using the arrow keys to move it and “break” whatever constrain there is.
(This is the way I have found to replicate the behavior every time - it’s not necessarily a ‘special’ object that does this.)
I think it’s normal behavior for SketchUp to be focussed on a plane the vertex is in. In your case the top face.
But if you have other geometry in a different plane (say a rectangle on the ground plane) and you hover the cursor over its face first (On Face), you’ll draw SketchUp’s attention away from your top face and you’ll be able to move the vertex to the ground plane (to face or to vertex) without hitting any arrow key.
You could also press the Alt key. This allows the geometry to deform by adding edges where necessary.
My experience of inferencing in SU 2016 is that it is rather more quirky than in previous releases. It is a deliberate development and works well most of the time, but occasionally can be a pain in the neck, thinking it knows what you intend better than you do. Not unlike printers and ink cartridges…
Unfortunately that doesn’t work - the best solution is using the [alt] key as Les said
It is by design Sketchup tries to not create new faces when moving things if not forced to. As les_boys said alt is used to toggle this behavior.