CPU Reports in Dashboard broken, when using a proxy host

TrueNAS Scale 21.08 Beta
TrueNAS Core 12.0-U5.1

If I access truenas via the ip like https://192.168.1.57/ui/dashboard and click on "CPU Reports" icon, I get the graphs displayed correctly.

But if I access the same report via my proxy host like https://myserver.com/reportsdashboard/cpu the graphs are not shown:

See also here:

https://www.truenas.com/community/threads/cpu-stats-not-working-in-12-0-u4.93525/page-4