You are probably getting your user variable you use in the code, before the signIn is actually finished. auth.user() will be null before signIn is completed.

Ah gotcha, yes I suspect that is the case, thanks for pointing it out! Is there any easy workaround?

It varies so much with the coding environment you are using, but you have not not use auth.user() or detect it is null and wait/error/signin before your user as completed signIn. I usually use console.log near the user() and signIn logic to see if the order is messed up. There are examples for a few environments on the Supabase page.