"The yellow ducks are not jumping out of the window but flying out."
Translation:A sárga kacsák nem kiugranak az ablakon, hanem kirepülnek.
Why isn't the "ki" prefix separated here? Usually it separated when the verb is preceded by a negation.
Because, here, the verb itself is negated, as opposed to something else in front of the verb being negated. So, the verb itself is the center of the action, therefore it stays intact.
There was a discussion at a similar sentence recently. It may help: