Disabling that option doesn’t show any highlight.
I’ve tried again. The keyword I told you has 3 characters. I also tried other words that have 3 characters and even 2 characters but no problem. The highlights show up correctly, no results without highlight. As I know for now It happens only the keyword I told you, it shows the result without highlight, strange. When it shows the result, there should be highlights on it. For example, it show 10 post results. The 6 results have highlights but the 4 results have no highlights but just show results and no keyword exists on the results.