Some people suggested possible thermal throttling on M1 machines, but I do not believe that it is correct.
My tests on similar machine shows full CPU usage under Maxwell Render or Houdini's Mantra.
One discovery:
If you have post processing options enabled during render, for example Bloom, Corona's CPU usage oscillates around 70% but can go as low as 50%. Perhaps some (all?) of the effects are single threaded and are interrupting render or the renderer itself needs to pause for a moment.
Perhaps you could re-do your test but with a plain render, all post-effects disabled, and tell us your results?
Have a nice day,