When I remove that line of code, it does cause the log error to stop generating, but I'm back at my original starting point (same as the below video), which you anticipated with the code you provided. I tested this new code in both my local environment (XAMPP, fresh installation with SSL active) and production environment (Sitegrounds hosting), and now it won't stay logged in on any pages that use auth.php for even the duration of the SESSION_LIFETIME.
Just in case, I have also attached my .env file with sensitive information redacted on my local testing environment. The /account folder contains the contents to the /public folder, and everything was set up to route correctly to the vanguard folder on the same level as the public_html. I have no idea if this is related, and it doesn't create any issues, but whenever I access the vanguard app (either directly or when calling auth.php), it recreates an empty public folder in its original location if the folder no longer exists. I doubt this is related, but was sharing it in case it somehow was.
Just trying to provide as much context as I can. Thank you for your support!