If the search api reports an error (e.g. "search backend too busy"), the app shows a misleading "No results found" and doesn't show the info reported by the api. This should be fixed, we had some tickets in our OTRS support queue, because of a search backend failure (related T102463 and the doc of it) with the question, why all search queries reports, that there aren't matches.
The api response was (i remember, that we're using search and not prefixsearch anymore, but i'm not sure :)):
{
"servedby": "mw1121",
"error": {
"code": "srsearch-error",
"info": "Search is currently too busy. Please try again later.",
"*": "See https://en.wikipedia.org/w/api.php for API usage"
}
}