The visual designer for .NET Core Windows Forms is still a preview build (after many years of continuous development),
https://devblogs.microsoft.com/dotnet/state-of-the-windows-forms-designer-for-net-applications/
It's just too huge a project to finish. If you want to learn workarounds, please refer to
https://github.com/dotnet/winforms/blob/main/docs/designer/readme.md
For projects it can load, this new designer is much cooler. However, when it fails on a project (like yours), it just looks bad.