It's returning a certificate for *.a.run.app rather than hn.algolia.com for some reason. I dug around a bit and you can (temporarily?) use this to access it: https://hn-search-web-main-3hmwcuxzqq-uc.a.run.app/
They're working on a new deployment and cert stuff probably got left off the list, as many of us (raising hand) can attest it sometimes does. It'll get fixed.
I was testing out something and ran into this issue yesterday as well. I created a cname to point to http://ip.jsontest.com/ so I could validate my DNS was working. For some reason I was getting the google 404 page.
That would be because that site is hosted on a Google service, but depends on the hostname to figure out which customer/backend to use. This is normal for any multi-tenant hosting. Your CNAME is not registered for that application in the hosting service's database and therefore they can't tell what it's for.
Remember that a CNAME isn't a redirect or anything, there's literally no way for the receiving webserver to know the domain that was CNAME'd in these sorts of requests.
Kind of strange how it’s been something like 17 years and HN still doesn’t have useful search built in. But also I guess kinda cool that you can build something that is popular for decades with little up no updates.
Edit: And it's back! Edit 2: And it's gone again!