same issue happened to me.
When trying to access to reports in groups that were created in the last few days we cannot do it. We're getting the same 401 error. Now, if we try to access to reports in groups that were created say 3 months ago, we have no issue.
Another thing to mention. If you login to Power BI portal with the same account you're using for accessing the reports and then try your application (in the same browser instance) you will be able to access to those reports.
It seems like extra cookies saved during the login to the Power BI portal are helpful in these cases. So, the problem is not is not the access token itself.