Compare the Material of the attached project.
If Decal is set to Emissive, it will not show up in thumbnail.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-130810 in the post.
|Component||UE - Rendering|
|Created||Oct 6, 2021|
|Resolved||Mar 23, 2022|
|Updated||Jan 26, 2023|