The website does say that it was ‘vibe coded’[0] so perhaps the author didn’t test it very thoroughly? They apparently do ‘vibe coding’ courses so.. that’s something.

[0] https://arcade.pirillo.com/

It's really impressive for what it is.

I tried it various pens and paper sizes and printer scales. And it suddenly worked but only if scanned at low res (200DPI).

Still I got a partially working font at the end