Invalid GUIDs on new material instance parameters cause overrides to break on base param rename

When a parameter value is assigned on a newly created material instance (constant or dynamic) using Set***ParameterValueInternal, the parameter’s ExpressionGUID is left invalid (0,0,0,0). Parameter linking relies on either the parameter name or its GUID. With an invalid GUID, any change to the base material’s parameter name causes the material instance override to lose its association.

If the material instance is later reloaded, the engine runs a fix-up that assigns valid GUIDs to parameters. After this reload step, overridden parameter values remain properly linked even when base parameter names are changed.

The issue only occurs before the first load/fix-up of a material instance, making it most visible when material instances are created and modified in code or through editor scripting without ever being reloaded.

1. Create a base material with a vector parameter named Color
2. Create a blueprint actor
3. Add a sphere mesh component to the blueprint
4. Create a custom event, called ApplyColor, that creates a dynamic material, for the sphere, from the base material, and sets the vector parameter value named Color to a new value.
5. In the custom events details panel, check Call In Editor.
6. Place this sphere blueprint in a level and click the ApplyColor button in the details panel.
7. Open the base material, rename the Color parameter to something else and click the Apply button.
8. Observe that the sphere color resets back to the base material color.
9. Rename the parameter back to Color and click the Apply button.
10. Observe that the sphere becomes the assigned Color value again.
11. Save and re-load the map with the sphere blueprint placed in it.
12. Repeat step 7
13. Observe that the sphere no longer resets back to the base material color.