The values you observed are completely 100% by design, but we understand that it has caused unnecessary concern for you and others, so I will provide additional explanation that should reduce that.
The spectrum analyzer process is running at low priority on just one of several cores, and it does not affect the data path (no performance impact). In fact, having spectrum data allows accurate on-channel and co-channel noise calculations for modulation changes, and enables smart channel change decisions. Turning off spectrum analysis is not a viable option for radio performance reasons, and we believe it is important enough to (mostly) dedicate a core for this purpose.
One proposed solution to alleviate the concern is to remove the included generic MIBs which report CPU utilization only on that CPU, but I have been advocating against that because the MIB also includes other more useful device health information. I'm afraid that might happen if users insist that this is a problem after being presented with the explanation.
We currently have no plans to expose all core utilization information in the Mimosa-specific MIB, but I will present that request. If accepted, it is likely to be much lower priority than other feature development in the queue.