Umlauts grouped in summary (U and Ü, A and Ä etc)
If I had to take a guess at this it would be that your database is using utf8 and the default collation of utf8_general_ci. If I understand correctly, this collation does not handle diacritics very well, and would consider U and Ü the same. You may have better luck with utf8_unicode_ci. That said, most of my Unicode experience is with Elasticsearch, so I know less about MySQL collation (assuming that is your database). You may have more luck running this by a database-specific stackexchange.

Here is a reference on the two collation types: https://stackoverflow.com/questions/1036454/what-are-the-diffrences-between-utf8-general-ci-and-utf8-unicode-ci.