Svelte/kit on CF behaves differently on two domains, with the same deployment

Hey all, I've deployed a svelte app using cloudflare pages, but it's behaving really really weirdly.
https://discord.com/channels/457912077277855764/1239235015422902372 - is my post in the svelte discord, but to simplify it, it works in the default pages.dev domain, but not with my custom menu.cruncho.app domain... and I've got no idea why