REST call from flutter to IPFS via HAProxy does not work

Background:

I have set up an IPFS server behind an HAProxy server. I have written a Flutter client to connect to this IPFS server through the proxy and add a file.

Problem:

Everything works as expected when I run the Flutter client as a desktop app (on Macos), but I get a 403 error when I run the Flutter client as a web app.

Details:

The headers for the successful call (captured via tcpdump) are as follows:

REQUEST:
POST /api/v0/add HTTP/1.1
Host: po.segito.net
User-Agent: Dart/2.11 (dart:io)
Accept-Encoding: gzip
Content-Type: multipart/form-data; boundary=dart-http-boundary-fmjPP-TIwnDcY7pJGniid4grt9mdDADtazmb7Pm8sP_PRJkV1oY
Content-Length: 39611

--dart-http-boundary-fmjPP-TIwnDcY7pJGniid4grt9mdDADtazmb7Pm8sP_PRJkV1oY
content-type: application/octet-stream
content-disposition: form-data; name="asset"

RESPONSE:
HTTP/1.1 200 OK
Access-Control-Allow-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
Access-Control-Expose-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
Content-Type: application/json
Server: go-ipfs/0.6.0
Trailer: X-Stream-Error
Vary: Origin
X-Chunked-Output: 1
Date: Sat, 07 Nov 2020 09:48:28 GMT
Transfer-Encoding: chunked

The headers for the unsuccessful call is as follows:

REQUEST:
POST /api/v0/add HTTP/1.1
Host: po.segito.net
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:82.0) Gecko/20100101 Firefox/82.0
Accept-Encoding: gzip, deflate, br
Content-Type: multipart/form-data; boundary=dart-http-boundary-HvcRoK0lGd4+oahuNaOejbIBd-cID5+0n3GFvwwOY4ZqfitRP1s
Content-Length: 39611

--dart-http-boundary-HvcRoK0lGd4+oahuNaOejbIBd-cID5+0n3GFvwwOY4ZqfitRP1s
content-type: application/octet-stream
content-disposition: form-data; name="asset"

RESPONSE:
HTTP/1.1 403 Forbidden
Content-Type: text/plain; charset=utf-8
Vary: Origin
X-Content-Type-Options: nosniff
Date: Sat, 07 Nov 2020 09:38:55 GMT
Content-Length: 16

403 - Forbidden

Notes:

1. There is no CORS issue because all files and REST calls (including the Flutter HTML files) are all served via the same HAProxy server.
2. The HAProxy is an SSL terminator.
3. A CURL request works as expected.
4. All other REST calls through the HAProxy works as expected.