Hi @Max, here is my -vv debug output connecting from Debian to Duo.
debug1: Local version string SSH-2.0-OpenSSH_9.2p1 Debian-2+deb12u2
debug1: Remote protocol version 2.0, remote software version dropbear_2020.81
I can only think your Debian machine is not routing the connection properly to the Duo. I am not sure how else it would see the SSH banner from the wrong O/S.
Could you try to telnet to the Duo on port 22? The response should show the SSH banner from dropbear.