404 Git diff of pull request

I have Github App installed on my private testing repository.
I am trying to get the actual git diff content of a pull request (not just the url).
If I access the diff endpoint in my browser, I get the expected output, so the endpoint, which pr.diff_url provides is correct. Important note is that when I access it from the browser, there is a token parameter added.

here is my authentication process:

auth = github.Auth.AppAuth(app_id, app_key)
gi = github.GithubIntegration(auth=auth)
installation_id = vars.private_repo_installation_id
try:
    installation = gi.get_app_installation(installation_id)
except Exception as e:
    print("Installation not found.")
    exit()
g = installation.get_github_for_installation()
repo = g.get_repo(<repo-name>)
pr = repo.get_pull(<pr-number>)

then, using the requests library:

response = requests.get(pr.diff_url)
if response.status_code == 200:
    diff_content = response.text
else:
    print(f"Failed to fetch diff content. Status code: {response.status_code}")

For public repos, it works fine, but for private repositories, I get status code 404.
I assumed it was the lack of authentication (although the status code is 404, not 403 or 401)
so I added:

headers = {"Authorization": f"token {gi.get_access_token(installation_id)}"}

and later in the request:

response = requests.get(pr.diff_url, headers=headers)

yet the 404 persists.

but I am unsure how to use it, couldn't find any examples in the documentation. The following code still encounters 404 error:

token = gi.get_access_token(installation_id)
token = token.token
url = pr.diff_url + '?token=' + token
response = requests.get(url)

Start by reproducing the private-repository case using the shown GitHub App authentication flow, then inspect how pr.diff_url and installation access tokens are handled. Done means identifying why the authenticated request returns 404 and documenting or correcting the supported way to retrieve the pull request diff content.