Since Joomla 3.7.3 the search component highlights the matched keywords not only in the article, but in the title too.
But, if there is a match in the title, the html tag shown in the results, instead the yellow highlight, etc:
This is the <span class="highlight">search keyword</span> in the title
The highlighting works fine on the default Protostar or Artisteer generated templates, so maybe it should be the T3 template which causes the problem.
If I comment out line 270 in components/com_search/views/search/view.html.php, the title highlighting is deactivated, but it's just a temporary solution.
Tested on Joomla 3.7.3, T3 2.6.5, T3 BS3 Blank 2.2.3