Can confirm in 1.17.1. Behavior was the same in 1.16.5.
Can confirm this "bug" in 1.17.1
My mistake, I believe I entered the wrong address when connecting. Can no longer reproduce.
This is not the same as MC-231339. The server is correct in disconnecting me in this report, it just uses the wrong message.
Both client and server were vanilla.
Can confirm in 1.17.1. Behavior was the same in 1.16.5.