This seems to be the design of material block properties.
When setting a material block property without an index, this overwrites the "Per Renderer" block, but when setting it with an index it overwrites the "Per Material" block.
So what's happening in the given project is that the animation is overwriting the "Per Renderer" value for "_EmissiveColor". When the provided script does the call of:
```
GetPropertyBlock(block);
block.SetColor("_Color", ...);
SetPropertyBlock(block);
```
this is reading the "Per Renderer" block which has the altered "_EmissiveColor". However when doing:
```
GetPropertyBlock(block, i);
block.SetColor("_Color", ...);
SetPropertyBlock(block, i);
```
This is reading the "Per Material" property block which does not have the changed value for "_EmissiveColor". The renderer seems to use the "Per Material" block if it exists, otherwise it falls back to the "Per Renderer" block, but it doesn't merge the blocks together. This can be verified by the adding some debugging akin to:
```
renderer_.GetPropertyBlock(perRendererBlock);
renderer_.GetPropertyBlock(perMaterialBlock, 0);
Debug.Log($"_EmissionColor: {perRendererBlock.GetColor("_EmissionColor")} {perMaterialBlock.GetColor("_EmissionColor")}");
```
And you'll see that the value is changing for the per renderer block, but not the per material block.

This could maybe use some improved documentation, or an option for animations to target a per material block instead of the renderer one.