By blocking TCP port 0 on Win10 it prevented it from doing something I didn't expect, I can't remember exactly what. It couldn't update itself or something with port 0 blocked. It's a rule I had always used before and I still block port 0 with my pf ruleset from my old Windows days
Read that some firewalls are configured to let port zero thru because the config tools assume there will be no port zero TCP traffic. Probably some monitors and net utils don't show TCP port0 either. Apparently pf blocks zero, but now it seems like a thing to verify.