The Search API is not guaranteed to be a complete index of all Tweets, and is generally optimised for recency rather than completeness - it does seem a little odd that none of that account’s Tweets are coming back in your query though. I wondered whether it was an issue with the underscore in the account name, but I’ve experimented and found that’s not the case since other accounts with underscores do return results.
I’m afraid I don’t have an answer for you on this one, other than that for some reason this account doesn’t currently seem to be hitting the public API Search index. I don’t know whether that account has previously been private or suspended, which might have been a signal to cause it to be excluded. Odd.