Surprising collation sort when using UpperFirst

I would expect that specifying UpperFirst would sort the capital before the lower, regardless of the following character. Do you have any insight?

```
:set -XOverloadedStrings
import qualified Data.Text.ICU as ICU
import qualified Data.Text.ICU.Collate as I
ICU.collate (ICU.collatorWith (ICU.Locale "de_DE") [I.CaseFirst (Just I.UpperFirst)]) "muller" "Müller"
-- LT
ICU.collate (ICU.collatorWith (ICU.Locale "de_DE") [I.CaseFirst (Just I.UpperFirst)]) "muller" "Muller"
-- GT
```