Azure AD Connect does not support the showInAddressBook attribute that CMN uses during sync objects to on premise AD (4319342)

Hybrid Active Directory (AD) setup of Coexistence Manager for Notes (CMN) and the Directory Connector will write the object to local AD and then Microsoft Azure AD Connect will synchronize the object into the cloud.  

The Rooms and Resource objects that are copied into local AD have all the required attributes for coexistence between Domino and local AD.

CMN syncs the object to on premise Active Directory and Microsoft Azure AD Connect copies supported objects and attributes to Microsoft 365.

Azure AD Connect does not support the 'showInAddressBook' attribute for Contacts synced to the cloud.

https://docs.microsoft.com/en-us/azure/active-directory/hybrid/reference-connect-sync-attributes-synchronized#exchange-online

Migrate the Domino objects that require 'showInAddressBook' to synchronize into the Microsoft 365 Global Address List (GAL) first, which would usually be Rooms and Resources objects from Domino.