You need to sync the ADatum users.

The SC-300 coverage of Azure AD Connect explains that when you need to bring in a new set of on-premises users or change what is synchronized (for example, adding an additional domain/OU such as ADatum or adjusting filtering), you reopen the Azure AD Connect wizard and choose “Customize synchronization options.” The guide notes that this path lets you “modify directory/OU filtering, add or remove forests, and enable optional sync features” so that only the intended users are synchronized. It contrasts with operational commands: Start-ADSyncSyncCycle merely triggers an immediate delta/full sync of the current configuration; Set-ADSyncScheduler changes the sync cadence or pause/resume behavior; and “Change user sign-in” alters the authentication method (e.g., PHS vs. PTA) but does not control scoping of which users are synced. To meet a requirement to sync the ADatum users according to technical constraints (such as OU/domain selection or feature toggles), you must first configure the scope by running the wizard and selecting Customize synchronization options, then perform/allow a sync cycle to bring those users into Azure AD as defined.