Wrong sorting in the Advanced Member Search

If you search for a member in the Advanced Search, the result list is not properly sorted.

1) Goto http://tinyurl.com/9utgqdq (Member Search on prod for user "Paul")
2) See the wrong sorting of the results, they seem not to be ordered alphabetically.

(happens only if there numbers in the strings of the results??)
It looks like this is connected to a problem with the fieldtype of the sort field in the solr schema.