Maybe try alias nano and LC_ALL=C type nano. Those test whether you have an alias or function named “nano” in bash that might be being run instead of /usr/bin/nano.
Oh, also, whoami and id. Maybe there’s something weird with how you’re logged in and despite not having the username “root” you’re still uid 1 or something strange like that?
Oh! Also maybe while you’ve got nano running, do a ps aux | grep nano and see which user is reported to own that process.