The system monitor sort order arrows are in the opposite direction to Dolphin. With the Breeze theme, in system monitor down is ascending and in Dolphin (and other Qt5 apps) up is ascending.
Created attachment 135734 [details] Looks good to me They're the same for me: bigger numbers on top and arrows pointing down. See attached screenshot (and ignore the fact that Telegram Desktop shouldn't be on top since it's smaller than other things; that's a bug).
I gather you see something different? Can you attach a screenshot?
Created attachment 135735 [details] Opposite sorting order arrow in original basic system monitor I was referring to the original basic system monitor application, not the new fancy one.
Created attachment 135738 [details] System Monitor and Dolphin sorting side by side comparison
Created attachment 135739 [details] Basic system monitor vs. new system monitor
Ah yes, the old one. Yeah.
ksysguard is no longer maintained, in Plasma 6 there is the Plasma system monitor for this task. If your issue still happens with the Plasma 6 replacement, please re-open and we can move this bug to the new product, thanks!