Use attached to reproduce. This was working in version 2017.1.221

Workaround:
private void Instance_LocationChanged(object sender, EventArgs e)
{
    var box = sender as RadMessageBoxForm;
    box.Location = new Point((this.Location.X + this.Width / 2) - (box.Width/2), (this.Location.Y + this.Height / 2 )- (box.Height/2));  
    
}