secure embed code &config has changed

I have been publishing links to my APP based reports on our Intranet using a URL obtained from the menu

  Share - Embed Report - Website or portal

as instructed by 

https://docs.microsoft.com/en-us/power-bi/collaborate-share/service-embed-secure

and
https://powerbi.microsoft.com/en-us/blog/easily-embed-secure-power-bi-reports-in-your-internal-portals-or-websites/

Note: these pages need updating as the menu link is no longer on the ... menu.

However I now get an error whan accessing the reports from the URL link

   Your Power BI session expired
   The current Power BI session can no longer be used to communicate with the Power BI service because of a service modification.    You must close and re-open your browser to start a new session.

The URL obtained from the above menu item looks like this, 

https://app.powerbi.com/reportEmbed?reportId=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&appId=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&autoAuth=true&ctid=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&config=12345678901234567890123456789012345678901234

If I go and get the URL today the &config is different to the one I got when I published my reports last year. This affects all my links.  They all show the same value as each other before and the ones I have checked all show the same new &config value now. The reportId and appId are the same as before.

What has happened? Is this a bug in POWER BI service? Have I got to go and chanhge all my links or is there a setting somewhere