Sorry Rebecca for the slow reply.

We have identified the issue and will be adding some minor documentation to pytool docs to help.
In short, nuget relies on new versions of mono.  Mono is not getting updated in linux distro maintained packages as fast as needed.

This resolved the issue for our Linux environments (not leveraging Azure VMs).
https://www.mono-project.com/download/stable/#download-lin-ubuntu

Please let me know how that works out for you.   This was never intended to introduce issues for local replication or other usage.

Thanks
Sean