Have you seen data missing in usage report in your tenant? I logged outlook.office365.com with my O365 mailbox on 8/17/2017 by Chrome and send a message. And now I check the Email app usage report. I can see some data in the report but I don't see my mailbox usage for Outlook on the web on that day. I am the Golbal Admin so it should not be permission issue. Has anyone come across this issue in your portal? Is it related to my subscription level?
Besides, the report data on Email app usage report is different from results returned by Get-ConnectionByClientTypeDetailReport. That's really confusing.
1. Why neither email app usage report in admin portal nor PowerShell get-connectionClientTypeDetailReport does not show my mailbox's OWA usage? Support: That's probably due to my browser Chrome and I am suggested using IE.
2. Why PowerShell get-connectionClientTypeDetailReport returns more users' OWA usage than email app usage report in admin portal? Support: Usage report may have its own logic to filter out some data. Belive in PowerShell if they are different.
However, that's not convincing me. It seems I cannot get a comprehensive answer from Support. If usage report and report PowerShell results have limitations (it would be great if you can confirm this with your dev team), I would suggest Microsoft write down in documentation and enhance in future.