Per mw:Manual:Hooks/UserLoginComplete:

This hook [--] is only intended for display purposes. Use the UserLoggedIn hook [--] for running backend logic on login.

However, due to its age, AFTv5 predates the UserLoggedIn hook, first introduced in MediaWiki 1.26, by several years, and thus AFTv5 uses UserLoginComplete to run literal backend logic on login in certain cases.

The hook handler is supposed to update the new(ly created) user's last feedback with their new user ID, provided that the code is able to extract an AFTv5 feedback ID either from the c URL parameter or from a cookie (ArticleFeedbackv5Utils::getCookieName( 'feedback-ids' )).

As long as the current approach works, I don't consider this a high priority item by any means, but rather something to look into on a rainy day in an attempt to improve the codebase's overall health and hygiene.