C# - Updatepanel Problem Or Possible Bug?

I have TextBox (multiline) and Label in an UpdatePanel which I refresh with javascript __doPostBack(upEditReminder,id);

Then I set both Label and TextBox text to current DateTime.

protected void upReminder_Onload(object sender, EventArgs e)
{
lbTest.Text = DateTime.Now.ToString();
tbReminder.Text = DateTime.Now.ToString();

Problem is that Label is updated but TextBox date is updated only once when the page is loaded but not when __doPostBack(upEditReminder,id); is triggered. I cant figure out what the problem is.

I have also tried textarea runat="server" but still have the same problem.