They have error monitoring, but download URLs are one thing that's tricky to monitor for "errors" correctly because if two URLs 404, how do you know from your Grafana dashboard or whatever which are valid and which aren't? Having run a server whose only purpose is to serve static files myself -- you get a lot of 404s, all the time, it's no indication anything is wrong at all. This case would only be picked up by a dashboard if it fatalistically caused an error somewhere (like a 500) but by definition this wasn't ever a 500, it's a 404.
As you note, it's just a bug. Sometimes things you don't understand might actually surprise you, it turns out.
