I am having the same issue with my Outlook Add-in in Outlook Classic on Windows where we are using a unified manifest. I submitted to Microsoft's QA forum and even a support ticket and neither helped get it resolved. In both cases, ultimately the support the people involved informed me that they did not have the necessary tools or access to be able to support further.

This is quite frustrating as it is a terrible user experience for our customers and is something that is not noted in Microsoft's documentation. I thought users would get the best experience by building with the latest approach (unified manfiest vs xml manifest) but it seems to not be the case.