Getting unexpected MISS when using Cache Reserve

We're using CDN with Cache Reserve to serve large binary files. I'm confused by the HIT/MISS behavior.

Our setup
  • We host large files on Google Cloud Storage in private buckets
  • We serve it through Cloudflare using a CNAME record on a subdomain cdn.ourdomain.com
  • We enabled Cache Reserve to also cache large files
  • We use GCS signed URLs to allow access to these files
  • We have tried with "Tiered Cache" either enabled or disabled, it didn't change the observations below
What I observe downloading a 2GB file from cdn.mydomain.com
  • It always returns MISS, no matter how many times i download it
  • However, after the first download, I see the "Egress savings (bandwidth)" and "Requests served by Cache Reserve" in the Cache Reserve page increase every time I hit the file, closely matching what I download
  • If I delete the file from GCS, I instantly get a 404 when trying to download the file again
Why I'm confused
  • Since the file seems to be served from Cache Reserved (according to the "Egress savings" metrics), I don't understand why I get cf-cache-status: MISS?
  • Why do I immediately get 404s right after deleting the file in GCS: does it mean Cache Reserve does a HEAD request every time the file is downloaded?
Trying the exact same setup with an 80MB file, I get HITs after the initial requests, and if I delete the file from GCS I can still download it until the cache expires.
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