How to get a consistent user id?

[vedantroy]

I have a hook in signIn that posts to my API server (I'm synchronizing AuthJS state with an API server's state):

		async signIn({ user, account, profile }) {
                                 // POST user.id to API server

However, what I've noticed is that the user ID provided in signIn is oftentimes different from the user id provided in the browser session through this callback:

		async session({ session, user }) {
			session.user.id = user.id;
			return session;
		},

How can I get a consistent user id on sign-in?