My frontend, registered to the playlistmoodevaluator.com domain on Cloudflare, routes to a React CRA-based app hosted on Render.
My backend, registered to the api.playlistmoodevaluator.com subdomain on Cloudflare, routes to a FastAPI python backend hosted on Fly.io.

The first request to my backend fails with Reason: CORS header ‘Access-Control-Allow-Origin’ missing, despite the fact that my FastAPI backend is configured with a CORS Middleware to accept requests from that frontend domain.

I ensured that my backend is configured to accept requests from my frontend, like so:

origins = [
    "https://playlistmoodevaluator.com",
    "https://www.playlistmoodevaluator.com",
]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

Here is a screenshot of the error:

As you can see in the screenshot attached below, this exact origin (https://playlistmoodevaluator.com), is expected by my backend’s Middleware and should be accepted.
This process was working until I registered my domain with Cloudflare and started routing my traffic through my new domain.