I just find some very very very weird thing. On my PC, when I output log, it shows a HEAD request with 403 error. But on backend, there is no HEAD requests at all, only GET requests.
My guess is Cloudflare turned my HEAD request into GET request, which causes the wrong signature (You change the request method, the signature will change as well).
Do you have any clue, why Cloudflare do this when Cache Everything turned on?