Replication fail with “There is no ‘username’@’host’ registered”

This post doesn’t contain a tip, I’m asking the q and I don’t know the answer yet. Could just be a logic error on my part, in which case it was just a personal mystery and I thank you for helping!
MySQL error 1449 “There is no ‘username’@’host’ registered”
You can get a replication fail with this [...]

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