The whole bar disappears for two reasons.

The screen touch lock is on. This is the common one. On a DMD device it is toggled by the assigned hardware button; on other devices there is a padlock in the top status bar. While it is locked the bottom menu and the zoom buttons are hidden and the map fills the height. The follow/compass button and the group button stay visible on purpose, so a rider using a remote can still see and change follow state. Unlock and the bar comes back exactly as it was.

You are in the route planner or Picture-in-Picture. Both take the bar away for the duration and give it back when you leave.

If the bar is there but a button is missing, that is a different setting. Settings → App Sections lets you drop Home, Devices or Roadbook from the bar so the ones you use spread out to fill the space. Map cannot be switched off, so the bar is never empty. Hiding a section only removes its button — the section still works, and its button reappears while you are in it.

If you are coming from classic DMD2: tapping the top status bar no longer toggles the bottom menu. That behaviour is gone, so an accidental tap up there is no longer the explanation.