I found the cause! I have levels in separate scenes, loaded additively. The World Space canvas in those scenes do not have a camera assigned by default, since the game camera exists in the main "game loop" scene.
As a test I imported my game level scene into your demo. It behaved exactly the same as in my game. So then I assigned a new camera to the game level scene and linked it to the World Space canvas. With this setup, the tutorial demo WORKS.
So I still need to do something about the cameras in the game, perhaps assign temporary ones and then switch them on load. I will keep you updated as this is perhaps important info to relay to people who buy your asset.