Just checked in a Firefox private tab. Same result.
The site statistics graphs are shown properly. The team statistics graphs are not shown.
An example team from our server:
Total Active Users
173
Public Channels
10
Private Channels
4
Total Posts
16484
Total Posts
Not enough data for a meaningful representation.
Active Users With Posts
Not enough data for a meaningful representation.
Regarding changes. We just update Mattermost with the Gitlab omnibus package monthly. I cannot say when the graphs stopped working. We also had a OS migration (Ubuntu 16.04 ->18.04) this year on the server, but againg, I cannot say if the behavior started after the migration or later.