Hello,

The issue is that on some machines Visual Studio fails to load the assemblies in the .NET Core server process from the NuGet fallback folder. As the behavior is inconsistent we have decided not to add our NuGet to a fallback folder to be used by the .NET Core designers. 

In order to use our .NET core designers and have the toolbox populated just install a Telerik .NET Core NuGet to your project. Then you can build it and open the form designer, the toolbox should be populated with the controls in the NuGet package. Additional information on the .NET Core designer: NET Core Design Time | Telerik UI for WinForms.

Regards,
Dimitar
Progress Telerik

Hi, Brian,

Thank you for reporting this case.

We will do our best to investigate whether it is possible to handle this scenario in an appropriate manner. Meanwhile, feel free to install the Telerik package in your project first and then add all the controls you need: https://docs.telerik.com/devtools/winforms/core/core-toolbox-version 

I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik