Get OAuth Custom Connector to check for valid token before initiating login flow again

This method you suggested works with a slight modification, namely instead of 

TokenMethod = (grantType, tokenField, code) =>
    let
        queryString = [
            grant_type = "authorization_code",
            redirect_uri = redirect_uri
        ],
        queryWithCode = Record.AddField(queryString, tokenField, code),

        tokenResponse = Web.Contents(token_uri, [
            Content = Text.ToBinary(Uri.BuildQueryString(queryWithCode)),
            Headers = [
                #"Content-type" = "application/x-www-form-urlencoded",
                #"Accept" = "application/json",
                "Authorization" = "Basic " & Binary.ToText(Binary.FromText(client_id & ":" & client_secret, TextEncoding.Utf8), BinaryEncoding.Base64)
            ],
            ManualStatusHandling = {400} 
        ]),
        body = Json.Document(tokenResponse),
        result = if (Record.HasFields(body, {"error", "error_description"})) then 
                    error Error.Record(body[error], body[error_description], body)
                 else
                    body
    in
        result;

I did 

TokenMethod = (grantType, tokenField, code) =>
    let
        queryString = [
            grant_type = "authorization_code",
            redirect_uri = redirect_uri
        ],
        queryWithCode = Record.AddField(queryString, tokenField, code),

        tokenResponse = Web.Contents(token_uri, [
            Content = Text.ToBinary(Uri.BuildQueryString(queryWithCode)),
            Headers = [
                #"Content-type" = "application/x-www-form-urlencoded",
                #"Accept" = "application/json",
                #"Authorization" = "Basic " & Binary.ToText(Text.ToBinary(client_id & ":" & client_secret), BinaryEncoding.Base64)
            ],
            ManualStatusHandling = {400} 
        ]),
        body = Json.Document(tokenResponse),
        result = if (Record.HasFields(body, {"error", "error_description"})) then 
                    error Error.Record(body[error], body[error_description], body)
                 else
                    body
    in
        result;

This does work in the sense that it returns results, but unfortunately it still has the same issue as the method I was using before where it requires reauthentication every time a query is changed, duplicated or refreshed and doesn't test the current token first before initiating the flow again.

Were you suggesting to combine this change in method with the Extension.CurrentCredential() thing? I can try that next and update if this change makes any difference (e.g. eliminates the errors I was getting previously with Extension.CurrentCredential()).

And yes this is all enabled for my role in the Netsuite UI, otherwise none of these methods would return results at all and would give 404 etc. (I went through that whole journey previously 😀) ... 

My general thinking is that the API calls are working fine, but it's actually the M logic that I have implemented that is causing the issue. I borrowed that and repurposed from other OAuth examples I found online, but maybe this Netsuite flow requires specific logic to get it to work. Like for example, I need to add an explicit check/reference to the current token or something...