I also use large and deep bowls of stainless steel.
So they throw almost no seeds on the floor.
But with the carrot and pepper sticks it's different.
They bite into the stick once or twice, throw the rest on the floor and take another one.
But I've never worried about that, because their natural habitat is a food surplus area. So their instinct tells them they don't have to save food.
Parrotlets are also South Americans. Perhaps it's the same thing as with linnies.
Another point is that you offer items that are something between shredding toy and food, for example the bird bread. That might encourage her to play with her food.
I can imagine that you are angry because you make such an effort to prepare their meals.
Does Gemma do this with all kinds of food?