SharePoint 2016 - Exchange 2016 - eDiscovery Failure

Currently running onPrem SharePoint 2016 Exchange 2016.
Attempting to configure eDiscovery on this setup however running into issues.

SharePoint 2016 Trusted Security Token Issuer for Exchange configured to https://*EXCHANGESITE*/autodiscover/metadata/json/1

Exchange Partner Application configured to https://eDiscovery.FQDN/_layouts/15/metadata/json/1

Authentication is currently running via ADFS SAML Claims using a Custom People Picker

Search Result source set to https://EXCHANGESITE/EWS/Exchange.asmx

When trying to select a mailbox to add to a case Error: The connection to the search service application failed.

When looking at the SharePoint logs: "Failed to connect to search serviceSystem.ServiceModel.FaultException....." "Exception of Type Microsoft.Office.Server.Search.Discovery.Internal.ExchangeConnectionException"

When looking at the IIS Logs on Exchange : "The user specified by the user-context in the token does not exist."

I'm at a loss at the moment. Exchange Auth Cert (confirmed by get-AuthCert on exchange) is on the SharePoint Servers.
SharePoint search works.
Permission controls via the custom people picker works.

Anyone have any suggestions?

If there is any need for clarification or additional information I am completely available to respond to comments. I am still having the issue.

Thanks