I also use a unique email address for Hacker News, and I get no spam to that address.

The (different) address I use for the HN jobs threads gets only spam, though.

I canary trap my email addresses too. No, I haven't had any spam on the one I use to login. The one mildly suggested in my profile about field isn't it.

Non credo che hacher news possa fare questo, ci tengono molto alla sicurezza altrimenti sarebbe un problema

Has anyone else experienced this?

Not I. I also use canary email aliases. Perhaps someone brute forced your login? Perhaps email dang and share what IP addresses/ranges your login should show up from and ask if any logins do not match those addresses. hn@ycombinator.com

Do you by chance use a HN specific UI App on your phone? The concept of those has always made my spidey senses tingle.