> And I don't think 'SS'<->'ß' is similar to the Turkish 'I with/without dot' problem, because the default Unicode mapping for 'ß' is correct in all languages, while the Turkish (and also Azerbaijani) problem is correct or broken depending on language setting.

I don't know if this counts as "correct" but it's still very confusing.

  >>> "ß".upper()
  'SS'
  >>> "ß".upper().lower()
  'ss'

  >>> "ẞ".lower()
  'ß'
  >>> "ẞ".lower().upper()
  'SS'
  >>> "ẞ".lower().upper().lower()
  'ss'