To reproduce:

Sub New()
    InitializeComponent()

    Me.WindowState = FormWindowState.Maximized
    Dim leftWindow As ToolWindow = New ToolWindow()
    leftWindow.Text = "Left Window"
    Me.RadDock1.DockWindow(leftWindow, DockPosition.Left)
    leftWindow.DockState = DockState.AutoHide
End Sub

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    If Me.RadDock1.DockWindows.ToolWindows.First().ParentForm IsNot Nothing Then
        Me.RadDock1.DockWindows.ToolWindows.First().ParentForm.Show()
    End If
End Sub

Workaround: maximize the form after the auto-hide popup is shown:
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    If Me.RadDock1.DockWindows.ToolWindows.First().ParentForm IsNot Nothing Then
        Me.RadDock1.DockWindows.ToolWindows.First().ParentForm.Show()
    End If
    Me.WindowState = FormWindowState.Maximized
End Sub