To reproduce:
- Add a groupbox to a form.
- Set larger than the default font.
- Add a grid to the group box.
- Dock it so it fills the available space.
- The grid is covering the bottom part of the header. 

Workaround:
radGroupBox1.Font = new Font("Segoe UI", 16, FontStyle.Regular);
radGroupBox1.Padding = new Padding(2, radGroupBox1.GroupBoxElement.Header.Size.Height + 2,2, 2);