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After deleting the node and readding it, I guess one of the earlier flags that I removed overrode its agent config somehow and that persists after deleting?

how are you deleting? https://docs.k3s.io/installation/uninstall

My first delete I had to use the k3s-uninstall.sh script, which implies to me something happened to treat it like a controlplane host. After I found that wasn't enough, I removed it completely with kube delete node and by doing that I was finally able to rebuild it as an agent.