I am having this issue now (Sept. 2017), although it was supposed to be fixed two years ago. I can search, find the keyword was looking for, but I cannot highlight the sentence unless I first clear the search term. I have (hopefully) attached a screenshot, where you can see the highlight tool is selected, the text is cursor-selected, but it doesn't highlight. I was able to reproduce the same error in a JSTOR and PLoS One paper, but not in a PeerJ paper. I am using Google Chrome.