Changing per layer parameters in a layered material that writes to RVT can cause temporary visual corruption

When using a layered material that writes to a runtime virtual texture.

Sometimes when modifying a layer parameter in the material instance editor.

The cached material data in the runtime virtual texture ends up with incorrect data.

It appears to only happen for the first few pages of the runtime virtual texture that are updated.

It may be hardware/driver specific (it has only been seen on NVidia). And is probably timing specific.

The corrupted data remains and is rendered until the page is evicted over time. This happens more quickly if r.VT.ForceContinuousUpdate is enabled. 

r.VT.Flush doesn't fully clean the corrupt data. But fully recreating the virtual texture by modifying r.VT.PoolSizeScale or using r.VT.ForceRecreatePool (in UE5.7) does.