If a layer has an input form of type=search, and jQuery UI Autocomplete behaviour attached, if the down key is used to select/focus a suggestion (resulting in the suggestion appearing in the box), a form scan event does not occur.
The form scan looks (as of 28cf184) for:
form#data #sections input[type="search"]').on ('input', function() {
which suggests that the autocomplete select/focus does not trigger the input event.
So far it has not been possible to find a way to force this to be triggered. References tried are:
https://stackoverflow.com/a/11168495/180733
http://api.jqueryui.com/autocomplete/#event-focus
https://stackoverflow.com/questions/19337877/trigger-event-once-dropdown-item-is-selected-on-jquery-combobox