I'm wondering what happens on landing.
From The Book, regarding VNAV approaches:
The FMC transitions out of "on approach" under the following conditions:
• selecting TOGA
• the airplane lands
• the airplane flies beyond the last waypoint in the approach (missed
approach waypoint or runway) and the VNAV page title changes from "ACT xxxxxx DES" to "ACT END OF DES"
It doesn't get any more specific than that, so I assume it must be a weight-on-wheels switch or tilt switch that gets rid of VNAV.