I mean technically Cloudflare is a CDN as well, there really isn’t a need for two in most instances… But I get it, I have customers with existing infrastructure, contracts, political issues so it’s not an unheard of configuration to use another CDN behind Cloudflare. Yes, it’s possible.
Ok, so usualsite.com is using Cloudflare. And it works. Because of the sample domains names I’m a bit unclear. Is different domain.com using Cloudflare? Is it on Fastly? Do you own/control both usualsite.com and differentdomain.com?
For the moment I am going to assume that you manage both domains and differentdomain.com is on Fastly based on the question. In Fastly are you overwriting the host header to subdomain.usalsite.com? If you do a curl direct to Cloudflare in this instance bypassing Cloudflare, does it return an error?
curl -Ikv -H “Host: subdomain.usualsite.com” https://differentdomain.com/specialfolder/ --resolve differentdomain.com:443:your.cloudflare.ip.for.subdomain.usualsite.com
(Might double-check the -H syntax, but pretty sure the rest is correct).