I've asked the same question on Stack Overflow: https://stackoverflow.com/questions/69455514/gcvirtualcontroller-not-displaying-with-skscene

It appears the issue only occurs when pushing from one ViewController to the GameViewController. When the GameViewController (with GameScene) is the entry point on the Storyboard the virtual controllers display as expected but if a different ViewController is the entry point and then the GameViewController is shown from that ViewController the issue occurs.