To reproduce:

1.Add a RadForm with RibbonFormBehavior.
2.Add a RadDock with one DocumentWindow and a button inside the ribbon.
3.Create another RadForm with RibbonFormBehavior.
4.Use the following code snippet:

public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();
    }

    private void radButtonElement1_Click(object sender, EventArgs e)
    {
        this.IsMdiContainer = true;
        this.radDock1.AutoDetectMdiChildren = true;
        
        ChildForm form = new ChildForm();
        form.Text = "MDI Child 1"; 
        form.MdiParent = this;
        form.Show();
       
    }
}

Workaround: Set RadDock.AutoDetectMdiChildren property to false.