REST call from flutter to IPFS via HAProxy does not work - flutter

Question

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