When I download resident key from my Solokey (or Yubikey, does not matter), 'ssh-keygen -K' in creates id_ed25519_sk_rk{,.pub} files (in my case, the _rk suffix would be used for other key types as well).

However, ssh with default IdentityFile configuration then skips the file because it is not listed in ssh/readconf.c#fill_default_options() and I have to explicitly add it to my ssh_config.

I'd say the *_rk files should be added to the default list.