In our Control, we have opened a new form to update the content of our control in the Designer. The new Form is not shown in the NetCore but works properly in the NetFrameWork. I have replicated the same issue in a simple control.

    [ToolboxItem(true)] // This attribute makes the control visible in the Toolbox
    public class CustomButton : Button
    {
        // Constructor
        public CustomButton()
        {
            // Set default properties
            this.BackColor = Color.Blue;
            this.ForeColor = Color.White;
            this.Text = "Custom Button";

            Form FileLoadForm = new Form();
            FileLoadForm.Controls.Add(new Button());
            FileLoadForm.ShowDialog();
        }
    }

We need the new Form should show while drag and drop the Custom control.