Replacing MeshMaterial3d to that of a custom material (MaterialPlugin) has no effect

## Bevy version

On main 35f1af522a0c13742f13abf5af04a3963463ad4d
Also tags v0.16.0, v0.16.1, varying results, see below

## \[Optional\] Relevant system information

```
AdapterInfo { name: "AMD Radeon RX 580 Series (RADV POLARIS10)", vendor: 4098, device: 26591, device_type: DiscreteGpu, driver: "radv", driver_info: "Mesa 24.2.8-1ubuntu1~24.04.1", backend: Vulkan }
```

## What you did

Code example https://github.com/tville/bevy/pull/1

In this minimalistic example, we first insert two cubes with a solid StandardMaterial, and later try to replace the materials on these cubes with a custom material (initiated with MaterialPlugin) with alpha blending, image, and color tinting.

## What went wrong

Over tags v0.16.0 and v0.16.1, for me, only one of the two cubes get to the correct state, depending on which MaterialPlugin is added first. This corresponds to a problem I'm having in my personal project.

When run against current dev/main version, none of the cubes get updated with the custom material.