The issue reproduces if the panels are collapsed before auto hiding the windows. When the window is later displayed the collapsed panels cannot be expanded. Check the attached video: 

Workaround:
Cache the old bounds.
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        
        this.radDock1.AutoHideWindowDisplaying += RadDock1_AutoHideWindowDisplaying;
        this.radDock1.AutoHideWindowDisplayed += RadDock1_AutoHideWindowDisplayed;
        this.radCollapsiblePanel1.PanelContainer.BackColor = Color.LightGreen;
    }

    Rectangle cache;
    private void RadDock1_AutoHideWindowDisplayed(object sender, DockWindowEventArgs e)
    {
        this.radCollapsiblePanel1.OwnerBoundsCache = this.cache;
    }

    private void RadDock1_AutoHideWindowDisplaying(object sender, Telerik.WinControls.UI.Docking.AutoHideWindowDisplayingEventArgs e)
    {
        this.cache = this.radCollapsiblePanel1.OwnerBoundsCache;
    }
}