Hi Lin Dan
This is a known challenge when access reviews are configured to use managers or specific reviewers who later leave the organization. Unfortunately, there isn't currently a native bulk "reassign reviewer" function in the Microsoft Entra admin center for active review instances. However, Microsoft does support configuring fallback reviewers through Microsoft Graph when using manager-based or owner-based reviews, which helps prevent reviews from becoming blocked when the primary reviewer is unavailable.
For reviews that are already in progress and stuck, the most scalable approach is typically to use Microsoft Graph Access Reviews APIs to identify the affected review instances and update or recreate the review configuration with replacement reviewers. Many organizations automate this process through Graph PowerShell SDK scripts, especially when multiple reviews are impacted.
I would recommend configuring fallback reviewers or designated delegate reviewers wherever possible. This ensures that if a manager account is disabled or removed, the review can still be completed without manual intervention. Microsoft Graph supports both primary reviewers and fallback reviewers as part of the access review definition.
If the reviews are already active and stalled, you may need to either update the review definitions via Graph API where supported or create replacement review campaigns and close the affected reviews, depending on the review type and current state. I'd also recommend reviewing your onboarding/offboarding process to ensure reviewer assignments are automatically updated when management changes occur.