Edit: Sorry, the previous description was technically incorrect. I have updated this with a better one.
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Currently the user name picker only matches the beginning letters of a whole word. For example, you can find "John Smith" by typing "Jo" or "Sm", but not "th" even though "th" is a part of "Smith". This is not a problem with Western names.

This, however, doesn't work well with East Asian names (Chinese, Japanese, Korean) in which the family name and given name are not separated by spaces. For example, if there's a person with family name "张" and given name "无忌", the full name appears as "张无忌". Now you can't find this person in the user name picker by just typing his given name "无忌". You must type "张" to find him.

As a workaround, we have to add a white space into the user name and make it look like "张 无忌". This makes it searchable, but is against the language. The extra white space also looks really weird when appearing in sentences which doesn't have word separators at all.