It depends which mode Order: POS Menu > "Which categories are displayed on POS Menu" is set to:

• Primary Categories mode: each meal is grouped by its own Primary Category field (a dropdown on the meal's edit page, listing your POS Categories). If a meal has no Primary Category chosen, it still appears on the terminal, but only under the permanent All Meals tab — never under a specific category tab.
• Meal Categories mode: meals are grouped by their normal Meal Category instead, and the meal's own Primary Category field is ignored entirely. In this mode a Meal Category's tab won't appear at all unless that category's "Show in POS terminal" toggle (on the Meal Category's own edit page) is turned on — so a meal can seem to be missing from the terminal because its Meal Category itself is hidden, not because of anything set on the meal.