I have a four-way stalk adapter of my own making: up/down volume and next/prev track.

In menus activated in MP3 mode, next/prev track act as next/prev item in the menu.

In menus activated in tuner mode, next/prev track have their normal effect of changing the tuned frequency, "through" the menu in the same way that volume controls act "through" the menu.

This behaviour is unintuitive and confusing, and I believe it's a bug.