Installing your test Manifest I see the following:
In the Add-ins button:
![enter image description here](https://i.stack.imgur.com/LQeZU.png)
In The ellipsis I see:
![enter image description here](https://i.stack.imgur.com/9u337.png)
So it's working for me in OWA, and I also saw similar results in Win32 Desktop Outlook. Are you on a private exchange server? Does your manifest work on an Outlook.com account?
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