This is reproducible with soma integration b18 with prism enabled. This is seen on mac 10.5.

Run the below program with prism. It shows a stage with 'fullScreen' set to true. But a normal Stage is shown - it is not full screen.

import javafx.stage.*;
import javafx.scene.*;
import javafx.scene.control.*;

Stage {
    fullScreen: true
    width: 250
    height: 250
    scene: Scene {
        content: ProgressBar {
            progress: bind ProgressBar.computeProgress(100, 10);
            translateX: 30
            translateY: 100
        }
    }
}