user object is undefined inside signIn callback of auth.js provider

Environment

OS: Linux 6.4 Fedora Linux 38 (Workstation Edition)
Node: 18.17.0
Brave Browser: 115.1.56.20

Reproduction URL

https://codesandbox.io/p/sandbox/authjs-solidjs-hd4x9n

Describe the issue

In auth.js package the user object received as prop in signIn callback is undefined. This means we can't do custom checks on the user signing up or logging in, or mutate the user object for a client that is signing in for the first time(signing up), for example a populating a required database field with randomly generated value.

How to reproduce

Go to this link:- https://codesandbox.io/p/sandbox/authjs-solidjs-hd4x9n

Do the setup and all that. You will need to populate github client id and secret in .env file. After that click the sign in button and check the console. User object will be undefined.

Expected behavior

Go to this link:- https://codesandbox.io/p/sandbox/next-auth-nextjs-kwkj6s

Same thing as above but this time user object is logged normally and it isn't undefined.

So ultimately the next-auth package works while auth.js package doesn't.

Start with the auth.js SolidJS reproduction at https://codesandbox.io/p/sandbox/authjs-solidjs-hd4x9n and compare its signIn callback behavior with the working Next.js example at https://codesandbox.io/p/sandbox/next-auth-nextjs-kwkj6s. Configure the GitHub client credentials, sign in, and inspect the console; done means the callback receives the user object as it does in the working example.