I think I found an issue. I noticed this issue under 2.4.4 beta2 and 2.4.4 release version.

Open a fresh browser tab. Add a product to the shopping cart. Click "My Account" link on the bottom (/index.php?main_page=account), it will display the login page (/index.php?main_page=login), which is correct.

Then go back to the shopping cart, click "Go to checkout", then click "Go to checkout" button under the Guest Checkout section. You don't need to fill anything. Go to the bottom of the page then click "My Account" ((/index.php?main_page=account)). You will see error "ERROR_GUEST_CHECKOUT_PAGE_DISALLOWED" on the top of the page. A log file will be generated as the following:

[08-Dec-2022 14:25:27 America/New_York] Request URI: /index.php?main_page=account
#1 checkout_one_observer->messageCheck() called at [/includes/autoload_func.php:61]
#2 require(/includes/autoload_func.php) called at [/includes/application_top.php:222]
#3 require(/includes/application_top.php) called at [/index.php:25]
--> PHP Warning: Use of undefined constant ERROR_GUEST_CHECKOUT_PAGE_DISALLOWED - assumed 'ERROR_GUEST_CHECKOUT_PAGE_DISALLOWED' (this will throw an Error in a future version of PHP) in /includes/classes/observers/class.checkout_one_observer.php on line 258.