When you assign a Preview Mesh to a Material Instance, save and close the instance, on reopening instance, the Preview Mesh will not retain connection to the Instance and will revert back to its assigned Material until a Instance value is changed, the Parent Material is reassigned, or the Preview Mesh is reassigned.
Also reproduced in 4.9 Preview 3 (Launcher) and Main-Promotable-CL-2649225
Before Closing and Reopening, Beachball assigned:
After Reopening Material Instance:
RESULTS: The Beachball has reverted to it's original, assigned Material
EXPECTED: The Beachball mesh in the Material Instance should show the Material Instance applied to it.
Head over to the existing Questions & Answers thread and let us know what's up.