Given:
[4] pry(main)> "σπυρος".downcase
=> "σπυρος"
@chrispanag a trivial workaround for you is simply to change the username to σπυρος, username and username_lower will be exactly the same and this will simply work.
I am mixed on adding workarounds to core just for this specific case, especially when a totally trivial workaround exists.
Additionally you could ban the use of Σ in usernames using our allowed unicode username characters setting which would ensure this issue never pops up.
All for fixing Ruby and Postgres here, but this is one long multi year battle to get these things fixed.