Every virtual box I’ve ever used has had a flickering robot graphic, I have no idea what causes it, and across the 4-5 devices ive seen URSims installed on, I have never heard of a fix. I’m afraid it’s just something to live with!
Not a fool, I tried to fix it first timer i ran a URSim, but i never had any luck! If on the off chance you do find something, I’d love to know what you did to fix it!!
I don’t know if this fix will be the same for you, but using VirtualBox 6.1.18 with the Guest Additions CD Installed and enabling 3D Acceleration fixed the Flickering Issue for me.