Media Plate with multiple Materials changes Element 0 to the default material if 'Use Media Plate Default' is selected for any other element

Media Plate isn't a feature of 5.0.3 so this is not a regression.

Media Plate with multiple Elements changes Element 0 to the default material if 'Use Media Plate Default' is selected for any other element.

1. Create or open any project
2. Open any Static Mesh from the Content Browser
3. Add a Material to the Material Slots of the mesh then save and close the Static Mesh
4. Drag a Video File into the Scene(it should become a Media Plate)
5. Select the Media Plate and navigate to the Details Panel
6. Set the Mesh to Custom and set the Static Mesh to the mesh with multiple Materials
7. Scroll down to the Materials then set each element to a different non-black material
8. Select Use Media Plate Default under Element 1

Expected Results:
The Material of Element 1 changes to MI_MediaPlate_X

Actual Results:
The Material of Element 0 changed to MI_MediaPlate_X