This seems to be a thing that others have experienced:
https://discuss.zerotier.com/t/solved-four-devices-but-two-of-them-cannot-see-each-other/1459/3
https://discuss.zerotier.com/t/inability-to-connect-to-one-machine-but-others-are-fine-ubuntu/1795
https://discuss.zerotier.com/t/cant-ping-devices/1144
There is a temporary solution in the first link. It did not work for all clients. I’ll try to “refresh” all the client in the network. Periodically doing this would seem impractical beyond a certain network size though. Has an bug/issue been filed for this?