To reproduce:
add two ToolWindows and several DocumenWindows. Use the following code snippet:

public Form1()
{
    InitializeComponent();

    this.toolWindow1.AutoHide();
    this.toolWindow2.AutoHide();
    while (this.documentTabStrip1.TabPanels.Count > 0)
    {
        documentTabStrip1.TabPanels.Remove(documentTabStrip1.TabPanels[0]);
    }
 
    TabPanel tabPanel = new TabPanel();
    tabPanel.Text = @"New Tab";

    documentTabStrip1.TabPanels.Add(tabPanel);
}

Run the application and hover one of the Autohidden ToolWindows.

Workaround: do not add TabPanel. Use the desired DocumentWindow or ToolWindow instead