When you place a Child Actor into Static Mesh Actor Derived BP then place that BP into another BP of the same class as another child actor, the thumbnails of all Blueprints in your project will included portions of the Final BP.
Also Reproduced in Main Promotable-CL-2593205
RESULTS: BP from the entire Project will get the meshes assigned in B and C in the Thumbnail Previews. (If the BP Thumbnails don't show it immediately, you can go into edit Thumbnail mode and it will appear or simply open and save BP again.)
EXPECTED: Not that, other BPs Thumbnails should be unaffected.
Thumbnails Not Showing Properly on Child BPs
Thumbnails Not Showing Properly on Child BPs
Preview Icon Bug? Seeing added skeletal mesh in every icon?
Actor Thumbnails bugged when childactor1 uses childactor2
Content Browser BP previews show other BPs Child Actors
Strange visual glitch in blueprint thumbnails
Head over to the existing Questions & Answers thread and let us know what's up.