Making some further observations, when I set processes = 12, for example, I can see 12 separate python processes + 4 additional processes also created which I assume are setup for the manager and, perhaps, other purposes.

Now, what makes these 4 additional processes interesting is that for me, multiprocessing.cpu_count() returns 24. And, it is when I set processes = 20 that the python code will sometimes terminate successfully. 20 + 4 = 24...so I am using every single cpu in that situation.

However, as noted, when I set processes = 19, it will always terminate successfully. 19 + 4 < 24...there is at least one cpu not assigned any work.

Perhaps there some some kind of race condition or swapping around of data structures or something that only happens on macOS when every cpu is in use by python for this purpose.