The two commands I ran above were going directly to your origin server (with an IP address 35.x.y.240), Cloudflare was not involved.
They are both for the same image.
The second request passed an Accept header which indicates that I can support webp images, and that the server should give me a webp image if it can. As I received back a file with image/webp content type this is definitive that your server is doing on the fly conversion of images.
You either have a cache on your server, or some other plugin that is doing this conversion.
I can also see a x-proxy-cache-info header coming from your server. I don’t know enough about Nginx caching, but that cache might need to be cleared to resolve the issue.