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.
The Material of Element 1 changes to MI_MediaPlate_X
The Material of Element 0 changed to MI_MediaPlate_X
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-174731 in the post.