Custom Connector OAuth2 On-Premises Data Gateway: Broken Redirect

Hello,
I have a custom connector which uses OAuth2 with my Django web app.
It works correctly with Power BI desktop and in the Visual Studio Code Power Query SDK.

I have set up an On-Premises Data Gateway to use it in the Power BI service, but I cannot get it to work.

In the Power BI service, when I choose "Edit credentials", either when creating a new connection to my gateway, or manually adding the datasource of my semantic model to my gateway:

• The popup window opens
• Redirects to "https://oauth.powerbi.com/views/oauthredirect.html"
• Redirects to my web app's login page, and I log in
• Redirects to
  "{base_url}oauth/authorize/?client_id={client_id}&response_type=code&redirect_uri=https%3A%2F%2Foauth.powerbi.com%2Fviews%2Foauthredirect.html&state={state}&scope=read&code_challenge={code_challenge}&code_challenge_method=S256"
• I authorize the Reading Scope
• It very briefly redirects to "https://oauth.powerbi.com/views/oauthredirect.html"
• It then redirects and gets stuck at "https://gatewayadminportal.azure.com/OAuthRedirect?code={code}&state={state}/

If I manually close the popup the connect button is greyed out.

I have tried changing my pbi_redirect_uri to https://gatewayadminportal.azure.com/OAuthRedirect, but then it never makes a request to my web app. The popup opens and closes, and then when I try to create the connection it makes a post request to my web app with no auth token.

Can anyone tell me where https://gatewayadminportal.azure.com/OAuthRedirect is coming from, and ideally how to make my connector work in the Power BI service?

Thank you

Additional info:
I do not have a Power BI premium license or premium workspace.
I am not using the (personal) gateway.

powerQRy.pq:

[Version = "1.0.0"]
section powerQRy;
//==== OAuth2 with PBI requires PKCE - https://learn.microsoft.com/en-us/power-query/handling-authentication ====
Base64UrlEncodeWithoutPadding = (hash as binary) as text =>
    let
        base64Encoded = Binary.ToText(hash, BinaryEncoding.Base64),
        base64UrlEncoded = Text.Replace(Text.Replace(base64Encoded, "+", "-"), "/", "_"),
        withoutPadding = Text.TrimEnd(base64UrlEncoded, "=")
    in 
        withoutPadding;

code_verifier = Text.NewGuid() & Text.NewGuid();
code_challenge = Base64UrlEncodeWithoutPadding(Crypto.CreateHash(CryptoAlgorithm.SHA256, Text.ToBinary(code_verifier, TextEncoding.Ascii)));
// ==== ====

oauth_client_id = "***";
base_url = "https://***/";
authorization_url = base_url & "oauth/authorize/";
token_url = base_url & "oauth/token/";
content_url = base_url & "api/power_bi/tables";
pbi_redirect_uri = "https://oauth.powerbi.com/views/oauthredirect.html";

[DataSource.Kind="powerQRy", Publish="powerQRy.Publish"]
shared powerQRy.Contents = () => 
    let
        headers = 
            [
                #"Authorization" = "Bearer " & Extension.CurrentCredential()[access_token],
                #"Content-Type" = "application/json"
            ],
        response = Json.Document(Web.Contents(content_url, [Headers = headers])),
        responseTable = Table.FromRecords(response, {"app", "name", "route_endpoint", "isLeaf"})
    in
        responseTable;

powerQRy = [
    TestConnection = (dataSourcePath) => { "powerQRy.Contents" },
    Authentication = [
        OAuth = [
            StartLogin = StartLogin,
            FinishLogin = FinishLogin
        ]
    ],
    Label = "powerQRy"
];

StartLogin = (resourceUrl, state, display) =>
    let
        AuthorizeUrl = authorization_url & "?" &
            Uri.BuildQueryString([
                client_id = oauth_client_id,
                response_type = "code",
                redirect_uri = pbi_redirect_uri,
                state = state,
                scope = "read",
                code_challenge = code_challenge,
                code_challenge_method = "S256"
            ])
    in
        [
            LoginUri = AuthorizeUrl,
            CallbackUri = pbi_redirect_uri,
            WindowHeight = 780,
            WindowWidth = 1024,
            Context = [
                code_verifier = code_verifier
            ]
        ];

FinishLogin = (context, callbackUri, state) =>
    let
        Parts = Uri.Parts(callbackUri),
        Code = Record.FieldOrDefault(Parts[Query], "code", ""),
        TokenResponse = Json.Document(Web.Contents(
            token_url,
            [
                Content = Text.ToBinary(Uri.BuildQueryString([
                    grant_type = "authorization_code",
                    code = Code,
                    redirect_uri = pbi_redirect_uri,
                    client_id = oauth_client_id,
                    code_verifier = context[code_verifier]
                ])),
                Headers = [#"Content-Type" = "application/x-www-form-urlencoded"]
            ]
        ))
    in
        TokenResponse;

powerQRy.Publish = [
    Beta = true,
    Category = "Other",
    ButtonText = { Extension.LoadString("ButtonTitle"), Extension.LoadString("ButtonHelp") },
    LearnMoreUrl = "https://powerbi.microsoft.com/",
    SourceImage = powerQRy.Icons,
    SourceTypeImage = powerQRy.Icons
];

powerQRy.Icons = [
    Icon16 = { 
        Extension.Contents("powerQRy16.png"), 
        Extension.Contents("powerQRy20.png"), 
        Extension.Contents("powerQRy24.png"), 
        Extension.Contents("powerQRy32.png") 
    },
    Icon32 = { 
        Extension.Contents("powerQRy32.png"), 
        Extension.Contents("powerQRy40.png"), 
        Extension.Contents("powerQRy48.png"), 
        Extension.Contents("powerQRy64.png") 
    }
];