I have run into a scenario where the Report Usage metrics are completely missing peoples' views activity long past the expected 24-hour data refresh period. I started to suspect an issue during a new workspace/app rollout, when I watched a user navigate to and view the report, but never even saw their name appear in the out-of-the-box usage metrics even days later.
For the given report, here is what I see in the out-of-the-box reporting (pulled via PBI Desktop to purposefully exclude the most recent date, since I would expect differences there): PBI Report Usage Dashboard.
However, if I download Power BI "viewed" records from the O365 Unified Audit Log, here is what it shows for that same report (filtered to the exact same date range, workspace, and report): Unified Audit Log Viewed Reports.
The additional people and views shown in the O365 Audit Log are much more in line with what I expected based on the trainings I have held and other communications with my users.
This makes me very concerned over why the Power BI Service report is so vastly different from what I am seeing in the Audit Log. I might expect some swings of numbers here and there based on potential differences in filters, refreshes, etc. between the two sources but this is a bit too much of a difference. I am a Power BI Service Administrator in O365, I am administrator over the workspace in question, and the report is not being viewed by way of any embed codes or external application. The missing users in question are either Workspace Members or have access to the report via a Power BI App, so all people are viewing the content from "within" the Power BI services (be it web or mobile).