Lost password link redirects to my-account/lost-password/,how to fix it back to default lost password

Try to put below code into your theme functions.php file

remove_filter( 'lostpassword_url', 'wc_lostpassword_url', 10 );

OR

function reset_pass_url() {
    $siteURL = get_option('siteurl');
    return "{$siteURL}/wp-login.php?action=lostpassword";
}
add_filter( 'lostpassword_url', 'reset_pass_url', 10, 2 );

Please let me know if any query.

Hope it will help you.