We are seeing the following on our Exchange 2016 CU4 box:
Log Name: Application
Source: MSExchange Certificate Deployment
Date: 25/04/2018 14:38:52
Event ID: 2005
Task Category: General
Federation or Auth certificate not found: CERTIFICATE REFERENCE #. Unable to find the certificate in the local or neighboring sites. Confirm that the certificate is available in your topology and if necessary, reset the certificate on the Federation Trust to a valid certificate using Set-FederationTrust or Set-AuthConfig. The certificate may take time to propagate to the local or neighboring sites.