I just noticed that the latest update that moves Python scripts to Python 3 will upset distributed polling if you do not install the Python 3 memcache library. Polling will still run but be unable to connect to memcache to coordinate the work, so all of the pollers will poll all of the nodes (at least this is what happened to me)!
The script that nags about Python 3 dependencies does not currently check this one. In Debian you can apt install python3-memcache. Or use pip.