JavaScript Change focus to password field login page being reset

This sounds like a timing issue, there is probably already some JS running after your script is ran.

Try wrapping your JS in $(window).load(), this will run the function after other page assets have loaded and should fix the timing issue.

For example…

$(window).load(function() {
    document.getElementById('user_pass').focus();
});

Here is some info about the difference between $(document).ready() vs $(window).load().