I had these issues aswell with some of my applications. It's not really an ideal way to solving the problem but you should try adding your own "Anchoring".
To do this you can use the Resize event on the MDI form and apply some calculations to your controls.
TextBox textBox1 = new TextBox();
textBox1.Size = new Size(mdiForm.Size.Width - 20, 25);
That way when the MDI form is being resized so will the controls. Again, not ideal.. but this is how i "fixed" it.