How to reproduce: check the attached project

Workaround: use a custom RadPanel control:
public class MyRadPanel : RadPanel
{
    protected override bool ProcessMnemonic(char charCode)
    {
        if (!Control.IsMnemonic(charCode, this.Text))
        {
            return false;
        }

        if (!this.Enabled || !this.Visible)
        {
            return false;
        }

        Control parentInternal = this.Parent;
        if (parentInternal != null)
        {
            if (parentInternal.SelectNextControl(this, true, false, true, false) &&
                !parentInternal.ContainsFocus)
            {
                parentInternal.Focus();
            }

        }

        return true;
    }
}