In the United States, and probably everywhere else, "breakfast in restaurant" (whether as a Welcome Gift option or as an alternative to a closed lounge) is supposed to include breakfast foods, beverages, and standard tips. This is even printed on standard breakfast vouchers.

I'll add an additional tip for exceptional service, but this is voluntary.

If a hotel is requiring guests pay an 18% surcharge on top of an elite breakfast, the hotel is cheating.

I've never seen this myself, and I assume it's rare. Most Marriott Bonvoy properties provide benefits properly.