Window closed() event function is never called

[sidwellr]

When creating a window, one of the event functions is closed(). But it never seems to be called. Here is a program that demonstrates this:

use nannou::prelude::*;

fn main() {
    nannou::app(model).update(update).view(view).run();
}

struct Model {}

fn model(app: &App) -> Model {
    let _win = app.new_window()
        .size(720, 720)
        .closed(window_closed)
        .build();
    Model {}
}

fn update(_app: &App, _model: &mut Model) {}

fn view(app: &App, _model: &Model, _entity: Entity) {
    let draw = app.draw();
    draw.background().color(STEEL_BLUE);
}

fn window_closed(_app: &App, _model: &mut Model) {
    println!("The window is closed.");
}

Run it, then close the window. The message "The window is closed." should be printed on the console, but isn't. A similar program for nannou 0.19.0 works correctly.