I found that the problem lies in
Engine\Source\Runtime\AssetRegistry\Private\AssetRegistry.cpp
UAssetRegistryImpl::UseFilterToExcludeAssets line 1179
AssetPackagePath.Mid(Path.Len(), 1) == TEXT("/")
The function was called at UAssetToolsImpl::GetAllAdvancedCopySources
\Engine\Source\Developer\AssetTools\Private\AssetTools.cpp line 823
I don’t know if it’s designed like this and whether it’s safe to remove this line.