IE mode - missing shared session or cookies between tabs (IE mode and Edge)

PZac2440 Hello!  Thanks for reaching out!  I'm not an expert at this but have you tried making modifications to the "shared-cookie host" portion?  

Based upon the documentation: Cookie sharing from Microsoft Edge to Internet Explorer | Microsoft Docs it says "The host attribute is used for host-only cookies (and a leading dot is an error). " and I see ".site1.com" in your example.  

Also, I'm not sure if it makes a difference but is there a reason the "shared-cookie host" and "shared-cookie domain" portion of your example has the same site ".site1.com"?  

If you are working on deploying MS Edge, have you heard of our FastTrack team (https://www.microsoft.com/en-us/fasttrack/microsoft-365/microsoft-edge)? They can help with configuring MS Edge for your organization and provide guidance with IE mode.  Thanks! 

-Kelly