Linux box is Fedora 39, latest ssh/openssl libraries. Remote box is Windows 10 LTSC build 17763, OpenSSH.Server~~~~0.0.1.0, sshd.exe claims 7.7.0.
Linux user1 tries to connect as ruser1@windows, where ruser1 is domain account domain\ruser1 and in the administrators group. Linux user2 tries to connect as user2@windows, where user2 is local account .\user2 and in the administrators group. Neither domain\ruser1 nor .\user2 have a ~/.ssh folder on the Windows box.
Both users' public ed25519 keys were added to the default c:\ProgramData\ssh\administrators_authorized_keys in the Windows server, and the OpenSSH SSH server restarted. ACL for the keys file was set as required.
Ssh for local user1, remote domain\ruser1 succeeds passwordless. Ssh for local user2, remote .\user2, requires password, with public key not accepted with a reply packet type 51.
Setting DebugLevel 3 on the Windows side, shows the correct rsa fingerprint being sent and it matches the entry to use in the administrators_authorized_keys file. There is nothing logged at this level that explains why.
What detail am I missing?