Detailed discussion here.
Try this instead for 'only' scrolling vertical.
(auto scroll needs to be false before it will accept changes)
mypanel.AutoScroll = false;
mypanel.HorizontalScroll.Enabled = false;
mypanel.HorizontalScroll.Visible = false;
mypanel.HorizontalScroll.Maximum = 0;
mypanel.AutoScroll = true;