You spawn too many subprocesses that finish virtually at the same time.
It leads to wakeup_fd overrun.

Python 3.6 is in security mode, sorry (and the fix is impossible).
Python 3.7 has warn_on_full_buffer=False flag for  https://docs.python.org/3/library/signal.html#signal.set_wakeup_fd but asyncio doesn't use it for reasons.

Python 3.8 doesn't subscribe for SIGCHLD by default, thus the provided example finishes without any warning.

I'm going to closing the issue as won't fix.