Slack API returning with invalid_auth when running GET from HTML?

I have created a Slack Bot in a Test workspace to play with. All I am trying to do is run a GET API to collect the content of a channel, but I am tripping up at the first hurdle...

The API = GET https://slack.com/api/conversations.history

Variables:

1. token= xoxb-xxxx...AwQ
2. channel= xxxxH9P
3. The Bot is added to the channel.
4. I have the correct Scopes added.
5. Using the Tester API here actually works!

But my own HTML code fails.. returning with invalid_auth... So I guess it must be an issue with my HTML... But the thing is, the exact same code works with another slack bot I set up previously!

The basic HTML:

    <html>
    <head>
    </head>
    <body>

    <button id = "SendButton" onclick="GetChannelFromSlack()">GET</button>
    <p id="GET API Output">Test</p>

    </body>

    <script>
    function GetChannelFromSlack() {
        var slackBotToken = "xoxb-xxxxAwQ";
        var slackChannelID = "xxxxH9P";

        var getSlackData = new XMLHttpRequest();
        var url = new URL('https://slack.com/api/conversations.history?token='+slackBotToken+'&channel='+slackChannelID);   
            
        getSlackData.open( "GET", url, true );
        //getSlackData.setRequestHeader = ('Authorization', 'Bearer xoxb-xxxxAwQ'); 
        getSlackData.responseType = 'json';
        
        getSlackData.send();
        
        getSlackData.onload = function() { 
            var jsonResponse = getSlackData.response;
            document.getElementById("GET API Output").innerHTML = JSON.stringify(jsonResponse);
        }
    }

    </script>
    </html>

(Code snippet won't actually work as I have not entered in my actual bot token / channel id)

Edit Additionally, to isolate the issue - here is a curl command which works for one Bot/Channel, but not the other:

Working: curl -X GET 'https://slack.com/api/conversations.history?token=xoxb-xxx-VKa&channel=CxxxxTX'

Not Working: curl -X GET 'https://slack.com/api/conversations.history?token=xoxb-xxx-AwQ&channel=Cxxxx9P'

I do not see any difference in the setup of the two bots...

If anyone can identify why this is failing I would be really appreciate it!

Regards, Dann