When you open a BP for the first time on each new editor session FAssetThumbnailPool::LoadThumbnail will eventually be called because in FAssetThumbnailPool::Tick, RealTimeThumbnailsToRender will hold an entry for the opened BP.
This will do ThumbnailTools::RenderThumbnail → UBlueprintThumbnailRenderer::Draw → FBlueprintThumbnailScene::SetBlueprint → FClassActorThumbnailScene::SpawnPreviewActor → UWorld::SpawnActor → …