Yes I’m aware of this. It’s not a bug, it’s simply that “prev30days” is not implemented for this report. It will work for some reports if you look closely.
However, I feel like I should just better handle this so I pushed this commit:
https://github.com/discourse/discourse/commit/c08dea386b46672be1e69a9ecb10d626bdd84d32
If there’s no prev30days data in the report, it won’t show the trending icons anymore. This commit has also an example of how to add this to a report. pr-welcome if someone wants to add it to other reports.