Blueprint Viewport not updating until BP reopened

Changing the Material on a Static Mesh Actor Blueprint is not updating in the BP Viewport until the BP is reopened. The Thumbnail updates immediately.

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Also Reproduced in Main Promotable-CL-2599540

1. Open QAGame Editor
2. Add a Sphere from Basic Shapes to the level
3. Click Add to Blueprint Button in Sphere's Details Panel
4. Save Blueprint
5. Open Blueprint in BP Editor (Make sure that Thumbnail is visible.)
6. Change the Material of the Sphere to any other Material

RESULTS: Thumbnail updates instantly, Viewport does not until you save and reopen

EXPECTED: Viewport should update as soon as material is changed.