Cannot add to Override Materials array of a Static Mesh Component

Materials cannot be added to the Override Materials array in a static mesh component because attempting to add elements to the array fails.

Regression:
Yes - using the plus button to add to the array works as expected in 4.15.2 (CL 3416026)

1. Open UE4 Editor (any project)
2. Add blueprint based on actor
3. Add static mesh component to the blueprint
4. With the static mesh component selected, expand Advanced options dropdown in the Rendering section
5. Click the plus button next to Override Materials

Result:
No elements are added to the array

Expected:
Pressing plus button adds an element to the array