eDiscovery - Issues exploring groups & users related to a hybrid data source

Hi all, first time posting - unusually I could find nothing out there that helped.
I work in an organisation has an on-premises domain which syncs to our tenant. I don't manage the domain or the sync, but I'm assured that the settings are vanilla and there are no errors being logged. 99% of our users are hybrid. The tenant is shared across multiple legal entities, so I'm using eDiscovery to fulfil our GDPR subject access requests

The issue I am hitting is straightforward. in eDiscovery searches with hybrid users as the data source, I cannot add related objects (manager, direct reports, groups the user is in). The properties are present in Entra, but not visible to Purview, so I'm not investigating sync errors at the moment.

For cloud objects, I can see manager, teams, etc. and it works fine. 

Does anyone have any insights they can share on the "explore and add" mechanics in eDiscovery search data sources? I'm drawing a complete blank on this one. Where should I be looking?