The account that sent that Tweet was only just created shortly before the Tweet was sent - 12 minutes earlier, in fact.
The public Search API is not a complete index of all Tweets (“it’s important to know that the Search API is focused on relevance and not completeness. This means that some Tweets and users may be missing from search results.”), and my guess is that this Tweet was not indexed for Search at this time.
You can of course still obtain the Tweet using the statuses/show endpoint.