rsa-sha2-256 interop/debugging help, anyone?

I'm taking another run at fixing my rsa-sha2-256/rsa-sha2-512
implementation.

Authentication fails.  But I have been unable to find out why.  The
only peers I have available for testing are work Linux machines,
running, of course, OpenSSH.  One of them is one nobody but me uses, so
I can fiddle with sshd_config; I set LogLevel to DEBUG, but even that
didn't give me any useful hints as to what I'm doing wrong.  Everything
appears to be going what I assume is normally...

Sep 21 18:05:32 mouse-devel sshd[254028]: Postponed publickey for mouse from 2607:f2c0:fffd:1020::2:93 port 49503 ssh2 [preauth]
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: userauth-request for user mouse service ssh-connection method publickey [preauth]
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: attempt 3 failures 1 [preauth]
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: temporarily_use_uid: 1050/1050 (e=0/0)
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: trying public key file /home/mouse/.ssh/authorized_keys
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: fd 5 clearing O_NONBLOCK
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: /home/mouse/.ssh/authorized_keys:4: matching key found: RSA SHA256:dqymZck/Y4aBcNhjHv3L8vqvM0/U9EAj2FwKwum4v2A
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: /home/mouse/.ssh/authorized_keys:4: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
Sep 21 18:05:47 mouse-devel sshd[254028]: Accepted key RSA SHA256:dqymZck/Y4aBcNhjHv3L8vqvM0/U9EAj2FwKwum4v2A found at /home/mouse/.ssh/authorized_keys:4
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: restore_uid: 0/0
Sep 21 18:05:47 mouse-devel sshd[254028]: debug1: auth_activate_options: setting new authentication options

and then it falls over with

Sep 21 18:05:47 mouse-devel sshd[254028]: Failed publickey for mouse from 2607:f2c0:fffd:1020::2:93 port 49503 ssh2: RSA SHA256:dqymZck/Y4aBcNhjHv3L8vqvM0/U9EAj2FwKwum4v2A

Does anyone happen to know of any way to coax useful info out of
OpenSSH, or, alternatively, does anyone have a different implementation
that _does_ give detailed failure info they'd be willing to do some
interop tests with me with?  Presumably I'm doing something wrong
somewhere but I'm having trouble figuring out what.  Everything works
fine if I turn on ssh-rsa and use that, but that just eliminates issues
with the things that aren't algorithm-dependent; it doesn't give any
hints what I'm doing wrong with rsa-sha2-256.

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