Requests seems to randomly not be logged in

We're experiencing that requests (sometimes up to 50%) made from the frontend forget that a user is logged in.

For endpoints that are explicitly authenticated in _staticwebapp.config.json_ this results in a redirect (due to fallback on 401 configuration) and for endpoints that allow anonymous there is no _x-ms-client-principal_ header once the backend is reached.

For the _.auth/me_ it either returns the expected JSON object or our _index.html_ (without redirecting).

I have no idea how to reproduce it. We've only experienced on one SWA instance and we have quite a few, but we experience it on all environments of that specific instance.

This SWA is a bit special, compared to our other instances, in that it is using an App Service as a linked API and custom Azure AD providers. But other than that nothing special.

I'm really at a loss as to what to try.