[SSAM]Textures that are still compiling are all counted as UnknownRef

In UE5.8, the texture scale fallback is no longer applied when SSAM is enabled. As a result, if a material cannot access its TextureStreamingData when it is loaded, its textures are treated as UnknownRef, which can easily overflow the texture streaming pool.

1. Create a unreal project
2. Add Starter Content. set Default Map to StarterMap
3. Add the following to Config/DefaultEngine.ini, then restart the editor:
   [SystemSettings]
   s.StreamableAssets.UseSimpleStreamableAssetManager=1
4. Once Launched. Run BUILDMATERIALTEXTURESTREAMINGDATA, then Save All. This saves TextureStreamingData into the materials.
5. Launch the editor, After stable loading, execute ‘stat streaming’ and see UnknownRef Mips (300MB+)
6. Launch the editor with -asynctexturecompilation=off. This forces textures to be built synchronously, guaranteeing that all textures are ready when their materials are loaded.
7. see stat streaming, make sure that UnknownRef Mips is under 10MB

Expected Result:

Even with SSAM enabled, almost no textures are counted toward UnknownRef Mips.