So I guess it has something to do with the query, I know that is supported special character, because I have notice that the search results are fine with special characters, but the highlight field is coming empty.
The highlight fields might not contain anything if there was no exact query match. You can use the fallback parameter to ensure something is returned every time. More on this behavior is outlined here: Highlight Fields | Swiftype Documentation.
I also want to note that I'm assuming you're working with Swiftype, and not Elastic App Search. Do correct me if that's not the case.
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