Regression: rendered texture not updating after change to Image

[bas-ie]

Bevy version

0.15.1

What you did

Minimal repro

use bevy::{
    asset::RenderAssetUsages,
    color::palettes::css,
    prelude::*,
    render::render_resource::{Extent3d, TextureDimension, TextureFormat},
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, init)
        .add_systems(FixedUpdate, draw)
        .run();
}

fn init(
    mut commands: Commands,
    mut images: ResMut<Assets<Image>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
    mut meshes: ResMut<Assets<Mesh>>,
) {
    let image = Image::new_fill(
        Extent3d {
            width: 256,
            height: 256,
            depth_or_array_layers: 1,
        },
        TextureDimension::D2,
        &(css::REBECCA_PURPLE.to_u8_array()),
        TextureFormat::Rgba8UnormSrgb,
        RenderAssetUsages::MAIN_WORLD | RenderAssetUsages::RENDER_WORLD,
    );
    let image_handle = images.add(image);
    let mesh = meshes.add(Rectangle::new(256., 256.));
    let material = materials.add(image_handle);

    commands.spawn((Camera2d, Mesh2d(mesh), MeshMaterial2d(material)));
}

fn draw(
    mut images: ResMut<Assets<Image>>,
    material: Single<&MeshMaterial2d<ColorMaterial>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    let Some(m) = materials.get(*material) else {
        return;
    };

    let Some(texture) = &m.texture else {
        return;
    };

    let Some(img) = images.get_mut(texture) else {
        return;
    };

    for x in 0..50 {
        for y in 0..50 {
            let _ = img.set_color_at(x, y, Color::WHITE);
        }
    }
    // This call is required to update render.
    materials.get_mut(*material);
}

Create an Image. Add it to a MeshMaterial2d, and render the material using a Mesh2d. In the FixedUpdate schedule, modify the Image asset.

What went wrong

The changes will not be rendered unless a call is made to materials.get_mut(&handle) after the changes have been made.

Additional information

It appears this may have been "re-broken" for quite awhile after being fixed in 107dd73. It's also possible that this is "working as intended" in the retained rendering world we live in, but I'm not flash enough to understand all the possible implications!

For some reason this behaviour is not evident in Update, only FixedUpdate. I have no idea why! I can also make it manifest in Update if I wait on mouse input e.g. by "drawing" on the image using the cursor, which smacks of a timing issue but no clue what.