There is a player named ü. The uuid is c6893ba47cfd4002abea33d4b1bd3f7a.
This is a legacy account that must have been created before usernames had proper restrictions.
It is possible to look this user up by uuid, but looking up by name (both u and ü) returns nothing.
Can you rename the account to u (and potentially other accounts with non alphanumeric chars) so it works properly with the API?
Also, are there accounts with usernames longer than 16 chars?