µMPX doesn't decide whether or not the CPU is throttled, this is a decision made by the OS (I'm guessing you're running Raspbian). You would see the same thing when running the command
in the terminal. You seem to have hit the "soft" limit (at 60 degrees C) which does not do as much throttling as the "hard" limit (85). If you don't see this on your other pi's, they must be cooler than this one or
are running an older Raspbian version which does not have the same throttling thresholds or
are not 3B+ models but 3B's.
See this link for more info: https://www.raspberrypi.org/documentati ... agement.md