The flow reporting success while permissions don't actually apply is a known pattern with this action. A few things to check given your exact situation.
First, check the service account connection inside the flow.
The Grant access action requires the connection to be authenticated as a list owner at runtime. If the service account's OAuth token expired or was silently re-authorized with reduced permissions around April, the action can complete without error but without actually applying the permission. Open the flow, find the Grant access action, click Change connection and verify the connection is still authenticated as the service account.
Second, check if your SharePoint tenant's sharing settings were tightened around April. Microsoft 365 admin changes to external sharing or people sharing settings can silently block the Grant access action from working even when the service account is in the Owners group. Go to SharePoint Admin Center > Policies > Sharing and check if the sharing level was changed to more restrictive settings around that time.
Third, check whether unique permissions are still enabled on the specific list or library. If someone reset permissions to inherit from parent on the list around April, item-level permission changes will silently do nothing because inheritance overrides them.
For logs: go to Microsoft 365 Admin Center > Reports > Audit log search, filter by the service account and the date range around April 1st, and look for SharePoint permission-related events. That will show whether the action fired and what actually happened at the SharePoint layer.
Best regards,
Valantis
✅ If this helped solve your issue, please Accept as Solution so others can find it quickly.
❤️ If it didn’t fully solve it but was still useful, please click “Yes” on “Was this reply helpful?” or leave a Like :).
🏷️ For follow-ups @Valantis.
📝 https://valantisond365.com/
