Outlook 365 Add-in disappears after a few days for some users, any fix ?

[SecureExchanges]

We developed an Outlook Office 365 add-in compatible with the "New Outlook" experience. It uses a valid manifest.xml, built according to Microsoft’s standards. Because the add-in uses the onSend event, we understand it cannot be published on AppSource, so we deploy it internally via the Microsoft 365 Admin Center.

The deployment works well for most clients. However, one enterprise client with over 60,000 mailboxes is facing a major issue:

After deployment to a small test group (200–300 users), the add-in randomly disappears after a few days for some users, even though it was successfully installed.

The deployment delay of 24–48 hours is expected per Microsoft documentation, but the add-in becoming unavailable or silently removed is the real issue.

We’ve opened several tickets with Microsoft. They ran a private diagnostic tool that temporarily resolved the issue, but the problem keeps returning.

The ticket was eventually closed with the following note:
"The diagnostic to Repair Company Object and Service Plan Failures was performed to resolve
the issue"

So far, no one at Microsoft has been able to provide a root cause or permanent fix.

Has anyone else experienced this with large-scale Outlook deployments?
Is there a known workaround or persistent solution?