It was answered in this thread: https://github.com/dotnet/aspnetcore/issues/60680

Key is to add explicit statements in .csproj:

<PropertyGroup>
    ...
    <StaticWebAssetBasePath>_content/RazorClassLibrary1</StaticWebAssetBasePath>
    ...
</PropertyGroup>

<ItemGroup>
    <None Include="wwwroot\**\*" Pack="true" PackagePath="contentFiles/any/net9.0/wwwroot/" />
</ItemGroup>