Despite the similarity in names, this module is not intended to be equivalent to Special:PrefixIndex; for that, see action=query&list=allpages with the apprefix parameter. The purpose of this module is similar to action=opensearch: to take user input and provide the best-matching titles. Depending on the search engine backend, this might include typo correction, redirect avoidance, or other heuristics.
You shouldn't even be seeing this. Bizzeebeever told me to add this here so that we can get responsiveness working for all screens.