In name lists with alphabets names with German umlaut are not shown correct. The behaviour is different depending which database is used.

PostgreSQL
user with first name / lastname with umlaut are not found until we add the umlaut in the alphabet in langconfig file

Maria DB
user with first name, last name with umlaut are added to a,o,u. Adding the umlaut in the alphabet the user is found in O and Ö.

We think this is a bug because it should never be possible that an enrolled user can not be found by this type of filtering. The reason behind are problems with postgreSQL understanding of diacritic signs.
postgreSQL offers an unaccent functions that may solve this problem but is not used. See https://www.postgresql.org/docs/current/unaccent.html

It was discussed here https://docs.moodle.org/dev/Database_collation_issue
and here https://moodle.org/mod/forum/discuss.php?d=427545 and https://tracker.moodle.org/browse/MDL-68274