Hm. Thanks!
Class "Autocomplete" add path as index.php?members/find&_xfResponseType=json. And on forums with enabled friendly urls it's path is some. That is not true or correct?
And when the unfriendly URL is hidden away in some AJAX request somewhere, the friendly URL is pretty irrelevant. The default AutoComplete URL is an unfriendly URL because it's hard coded in the javascript, and its done that way because the javascript has no access to a link building class or any concept of the XenForo options to determine if the friendly URLs are working or not.
You can pass any URL you like, friendly or unfriendly to the JS as a data-acurl attribute assigned on the input element.
I know that I can set any URL. I was interested in only the extent properly when the friendly URL for use AJAX links with index.php. It turns out that this is not a bug, but a feature. Well so be it. Although I do not seem right.