https://support.cloudflare.com/hc/en-us/articles/214534978-Are-the-HTTP-2-or-SPDY-protocols-supported-between-Cloudflare-and-the-origin-server- says that Cloudflare will allways use HTTP/1.x between origin server and Cloudflare, while we all know that HTTP/2.0 would benefit for performance (also while connecting origin).
Please same consideration also for HTTP/3 when it gets stable and officially available in customers servers (Apache, Nginx, etc.).