LoadContext.get_handle() returns invalid or dropped handles

## Bevy version

0.6.1

## Operating system & version

Windows 10

## What you did

1. Create a custom `AssetLoader`
2. Create sub-asset resources, such as `StandardMaterial`s, and save them to the current `LoadContext` via `load_context.set_labeled_asset(label, LoadedAsset::new(asset))`
3. Reference these sub-asset resources via `let path = AssetPath::new_ref(load_context.path(), Some(label))`
4. Get a handle to the resource at the path `load_context.get_handle(path)`

5. Spawn entities with these handles

Repro documented in https://github.com/agg23/bevy_loadcontext_example

## What you expected to happen

With the bug repro code above, we would expect to see a 10x10 grid of cubes of various colors (`StandardMaterial`s) every time the program is run:


## What actually happened

However, this complete grid appears rarely, and most of the time, many materials don't render:


Inspecting using `bevy_editor_pls` shows the entities as we'd expect, but the material handles don't resolve to anything:


Start with the linked bevy_loadcontext_example and trace the custom AssetLoader flow through LoadContext.set_labeled_asset, AssetPath::new_ref, and LoadContext.get_handle. Reproduce the issue on Bevy 0.6.1 and verify that spawned entities consistently resolve their StandardMaterial handles and render the complete grid.

Written by the indexing model from the issue text.