KLM replaced its 408-seat 777-300's on the Amsterdam (AMS) - Buenos Aires Ezeiza (EZE) - Santiago (SCL) route with 294-seat 787-9's in approx. mid-2019. The equipment change apparently continues to this day as the Dutch carrier has operated the type on its authorized "special flights" on this route several times recently (regularly-scheduled flights have not returned to Argentina yet).
The change effectively reduced the number of Economy Class seats operated by KLM to Argentina and Chile while maintaining the quantity of Economy Comfort and World Business seats as the 777-300's have 334, 40 and 34 seats in each class, respectively, while the 787-9's have 216, 48 and 30 such seating.
The economy class seat reduction might be to balance out the increase in number of flights by KLM on the route in recent years from 4x/weekly to 6x/weekly. The consistent number of premium cabin seating probably reflects the importance of business travel on this route.