This item has been identified as an outstanding known issue. A workaround is provided until the issue is officially fixed.
For versions 1.6, 1.7 and 1.8 of the PowerPoint Add-in elevated privileges are required to install the add-in through command line and for the "all users" option.
Installing through the UI for "single user" will avoid this scenario.
For mass deployment:
Open command Prompt as Admin or
Use the switch /qr for a "pseudo" silent install. This will install the add-in with a reduced UI that requires no end user interaction (so long as switches are present) while avoiding the elevated privileges required.
Msiexec.exe /i “<path to msi installer>” /qr
If this workaround is not suitable in your use case, please contact Support at firstname.lastname@example.org.