Can the documentation please describe "x-sveltekit-invalidated"

Describe the bug

I write a PWA and get an error "The FetchEvent for "https://.../__data.json?x-sveltekit-invalidated=1" resulted in a network error response: the promise was rejected." I googled x-sveltekit-invalidated and found nothing except two issues that seem unrelated. Should the sveltekit docs not describe this message somewhere?
Right now, I am quite disappointed that my PWA does not really work offline, as promised.

Reproduction

https://github.com/michaelu123/nk
I start the app with npm run preview, then start ngrok with port 4173, then I start the installed app on a phone, then I disable the network, then I click on a map marker to display more info about the location (all data being in IndexedDB), then I get the error. If I enable the network, display the data, disable the network, and display the data again, it works, for some time. But I want to understand what x-sveltekit-invalidated is about.

Logs

System Info

Android, App installed from result of npm run build, npm run preview, resp. npm run dev

Severity

annoyance

Start with the linked reproduction and its npm run preview, npm run build, and npm run dev flows, then observe the offline PWA request containing x-sveltekit-invalidated. Document what that message means and how it relates to the reported IndexedDB and offline interaction, with the explanation verified against the described Android reproduction.