In DIRSYNC feature Group-Based User Creation Requires Resync to Generate Person Document

When directory synchronization (Dirsync) uses group membership as a criterion for determining which users are synchronized, any change in group composition—such as users being added to or removed from the group—requires initiating a full resync.

This requirement exists because Dirsync identifies users for synchronization based on records that have changed since the last sync. However, when a user is simply added to a group, the individual user record itself remains unchanged. As a result, Dirsync does not detect this update automatically.
To ensure that all relevant users are accurately synchronized when group membership changes:
Perform a full resync whenever group membership criteria are modified.
Schedule a daily Dirsync resync as a temporary workaround to keep data consistent.
Note: Implementing automated detection of group membership changes would require an architectural enhancement.