Web dashboard login fails "Invalid username or password" — request never reaches self-hosted server

Self-hosted Jellyfin v10.11.11 behind a Cloudflare Tunnel, external URL https://[my-domain]. Credentials are confirmed correct — same server URL/username/password work fine logging into both the native Android TV app and the phone app. The web dashboard fails every time with "Invalid username or password" using those identical, working credentials. Diagnosed before reporting: during each failed attempt, I checked both my Jellyfin server logs and my Cloudflare Tunnel logs in real time. Neither shows any incoming connection attempt at all — no failed auth entry, nothing reaching the tunnel. Confirmed across multiple attempts on separate occasions. This suggests the request isn't reaching my server, and "invalid username or password" is likely a generic error masking a different failure — possibly specific to how the web client connects to self-hosted servers behind a reverse proxy/tunnel, since the identical URL works fine for the native apps. Is this a known limitation for self-hosted setups, and is there a workaround?