Custom Connector, Gateway & OAuth

I have a custom connector where OAuth works fine via Desktop. However, while attempting to implemement it via an On-Premise Gateway running in enterprise mode on WinServer '25 I'm running in a handshake failure in the Service when attempting to create my connection.

When I click "Edit Credentials" in the Create Connection dialog in the Service a secondary browser window flashes and closes, presumably this would be the window where I grant access. With some error handling, it seems that I'm not getting the auth code back from the redirect.

I'm not using PKCE as Atlassian does not support it, I I beleive that I could go the route of building an external app to handle the authentication to Atlassian, then just pass everything into Power Query but I was really hoping to avoid doing that. This connector is meant to stay "private" (I'm not trying to get it certified).

Here is the error message I get in the connection dialog:
"ConnectorTestbed: [NoCodeInCallBack] The OAuth callback URI did not contain an authorization code." 

For reference here is my finish login code where the error is occuring:

    FinishLogin = (context, callbackUri, state, OAuthConfig as record) =>
        let
            Parts = Uri.Parts(callbackUri)[Query],
                
            Code =
                try
                    Parts[code] otherwise Error.Record(
                        "NoCodeInCallback",
                        "The OAuth callback URI did not contain an authorization code.",
                        [ Parts = Parts ]
                    ),

            Result =
                if Record.HasFields(Parts, "error")
                then error Error.Record(
                    Parts[Error],
                    if Record.HasFields(Parts, "error_description")
                    then Parts[error_description]
                    else "OAuth authorization failed",
                    Parts
                )
                else if not Record.HasFields(Parts, "code")
                then error Error.Record(
                    "NoCodeInCallBack",
                    "The OAuth callback URI did not contain an authorization code.",
                    [
                        callbackUri = callbackUri,
                        QueryParameters = Parts,
                        AvailableFields = Record.FieldNames(Parts)
                    ]
                )
                else
                    TokenMethod(Parts[code], OAuthConfig)

Any suggestions would be helpful, I think I've trolled many of the sources on OAuth & Power Query, so hoping for some fresh eyes and feedback. As noted, this all works fine in the Desktop Environment. 😞