B2B Guest Invitations blocked - "Invitations are blocked for this directory due to suspicious activity"

We are running a multi-tenant SaaS application (ASP.NET Core) hosted on Azure (Canada Central region). Our application uses Microsoft Graph API (v1.0) with application permissions (User.Invite.All) to send B2B guest invitations to external users via POST /invitations.

Starting around February 10, 2026, all invitation requests began failing with the following error:

Code: Forbidden

Message: "Invitations are blocked for this directory due to suspicious activity. Please contact Microsoft support for help."
What we've tried:

- Verified External Collaboration Settings in Entra admin center — guest invitations are allowed

- Confirmed the App Registration has User.Invite.All permission with admin consent granted

- Verified the service principal and credentials are valid (other Graph API calls like GET /users work fine)

- Only a small number of invitations were sent (fewer than 10 total) — this is not a bulk/spam scenario

- The tenant is a production tenant under a paid Azure subscription (<PII REMOVED>)

Tenant details:

- Tenant type: Workforce (Microsoft Entra ID)

- Region: Canada Central

- Subscription: Pay-As-You-Go

- Number of guest users in directory: fewer than 5

Questions:

1. What triggered the "suspicious activity" block on our directory?

2. How can we get this block removed? There is no self-service option visible in the Entra admin center.

3. Is there a timeframe for automatic unblocking, or does this require manual intervention by Microsoft?

This is blocking a critical feature in our production application. Any guidance on resolution would be appreciated.