We recently stopped using email addresses when generating usernames during SSO.
https://github.com/discourse/discourse/commit/88ecb83382e4dc5cec1d9599619a6c40a07825ae
The recommended solution is to supply a username in the SSO payload. You can go back to the previous behaviour by changing a hidden site setting, but we might remove it in the future.
./launcher enter app
rails c
SiteSetting.use_email_for_username_and_name_suggestions = true