I have the same question in UE5.1.1. After reading the code about rendering thumbnails, I found the reason: I had added a new flag in showflag.cpp, which ultimately changed the lighting. In this case, I disabled the flag in this function, and now it works well. Maybe this can help someone.
// for rendering thumbnails, cascade, material editor, static mesh editor
// keeps bloom as material properties with HDR can be better perceived
void DisableAdvancedFeatures()