Hope you are doing well.
From my understanding Dynatrace is unable to download source code for .NET core libraries. At this point there is no workaround.
Since .Net core is open sourced by Microsoft I would recommend searching through their documentation if you know which classes you want to see.
Are you trying to use this feature to troubleshoot your application? It might be beneficial to pull the code that was developed by your team that implements .net core.