As @LundinCast pointed out, there seems to be an outage on the Google server side. I'll monitor the situation and mark this as resolved for now.
Edit: I'm guessing this is related to https://status.cloud.google.com/incident/cloud-networking/19016
I created a custom domain mapping for my Cloud Run service following this guide https://cloud.google.com/run/docs/mapping-custom-domains.
I can access my service via the https run.app URL and the custom domain via HTTP, but when I go to the custom domain via HTTPS, I get back a Google 404 error page.
The weird thing is, this seems to be an issue on my local laptop (both browser and curl on the terminal), but curl-ing it from a remote server seems to work ok.
As @LundinCast pointed out, there seems to be an outage on the Google server side. I'll monitor the situation and mark this as resolved for now.
Edit: I'm guessing this is related to https://status.cloud.google.com/incident/cloud-networking/19016