The issue is reproduced with jdk 14.0.1 and not reproduced with jdk 13.0.1

To reproduce the issue run the FontSizeSample JavaFX app below on Raspberry Pi with enabled Monocle:
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import javafx.application.Application;
import javafx.scene.text.Font;
import javafx.stage.Stage;
import javafx.stage.Screen;
 
public class FontSizeSample extends Application {
    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(Stage primaryStage) {

        for(Screen screen: Screen.getScreens()) {
            System.out.printf("screen: %s%n", screen);
        }

double fontSize = Font.getDefault().getSize();
System.out.printf("font size: %f%n", fontSize);
    }
}
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The result with jdk 14.0.1:
> java -Dglass.platform=Monocle FontSizeSample
----------------
screen: javafx.stage.Screen@74d6a0fb bounds:Rectangle2D [minX = 0.0, minY=0.0, maxX=1280.0, maxY=720.0, width=1280.0, height=720.0] visualBounds:Rectangle2D [minX = 0.0, minY=0.0, maxX=1280.0, maxY=720.0, width=1280.0, height=720.0] dpi:1280.0 outputScale:(1.0,1.0)
font size: 120.000000
----------------
The screen DPI is 1280 and the default font size is 120.

Run the same app with jdk 13.0.1:
> java -Dglass.platform=Monocle FontSizeSample
----------------
screen: javafx.stage.Screen@353aa0fb bounds:Rectangle2D [minX = 0.0, minY=0.0, maxX=1280.0, maxY=720.0, width=1280.0, height=720.0] visualBounds:Rectangle2D [minX = 0.0, minY=0.0, maxX=1280.0, maxY=720.0, width=1280.0, height=720.0] dpi:96.0 outputScale:(1.0,1.0)
font size: 16.000000
----------------

The screen DPI is 96 and the default font size is 16.