Login template uses S_LOGIN_REDIRECT for "redirect" hidden field, but this field can be also generated in S_HIDDEN_FIELDS value, so if you use link to login in phpBB and redirect to our page:
/phpBB3/ucp.php?mode=login&redirect=/my-page.html
We will get 2 "redirect" form fields, and the value from the URL won't be used.

The solution is to remove S_LOGIN_REDIRECT from the login_body.html template, but use in ucp.php

case 'login':
if ($user->data['is_registered'])
{
redirect(append_sid("{$phpbb_root_path}index.$phpEx"));
}

login_box(request_var('redirect', str_replace('&', '&', build_url())));
break;