Turn out that the cause is our misconfiguration. This is the line of code which check if the link is leading to outside:

referer is the URL of original page (where we saw the link and we clicked), like https://app.eventyay.com/talk/orga/event/moon-night-2025/mails/1/, request is the target URL,like https://app.eventyay.com/talk/moon-night-2025/me/submissions/QQSD3B/. With proper setup, the above check will pass, because referer.netloc == request.get_host() == 'app.eventyay.com'.

But with current configuration, even if we already enable TLS, the request.scheme is still http, not https and make the above check fail.