As I indicated in my comment, you've done something odd to your window, which is why you're getting this behaviour. I can reproduce this. I created a new WinForms project, put a button on the form that has this code:
private void button1_Click(object sender, EventArgs e)
{
this.WindowState = FormWindowState.Maximized;
}
Added a status bar, and set the MaximizeBox property of the form to false. Running the code, pressing my custom button, the form maximizes, but the status bar disappears behind the task bar.
If I then close the form, re-enable the MaximizeBox property, and re-run the project, then maximizing the form gets the behaviour you want (the status bar's bottom is aligned with the top of the task bar). So re-enable your MaximizeBox and you should be done.
(Once the Maximize box is available, maximizing works whether using that or the custom button)