Sounds like a broken web app.
You are currently using a webapp that doesn't do this. It's called Hacker News, and it never asks me to login every time on my phone.
> when the app can just cache my authentication and FaceID me
Sounds like a broken login form.
Hacker News also allows me to login with Face ID on my phone, thanks to my password manager.
Optionally webapps can also provide Passkeys.
>
> You are currently using a webapp that doesn't do this. It's called Hacker News, and it never asks me to login every time on my phone.
Every time I visit Hacker News on my iPad I'm logged out. Apple has decided that if you don't visit a website often enough it will expire all your cookies for the site.
In practice that means I can log in to HN while I'm at the cafe one weekend and be logged out by the time I visit the next weekend.
Apparently HN does it on purpose and kills alls sessions on all devices when you log off.