To reproduce:
- Create a custom column that uses RadTextBoxControlElement as permanent editor.
- Add the column to the grid. 
- Scroll the grid so the custom column is not visible.
- Put the the grid in edit mode and change the current cell.  

Workaround:
- Disable the IME support:
public class MyRadTextBoxControlElement : RadTextBoxControlElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTextBoxControlElement);
        }
    }
    protected override void OnLoaded()
    {
        this.InvalidateMeasure();
    }
}