Found this post when I was experiencing similar symptoms. A document created a few years ago, opened up a few years later on Word on my Surface. The document had sporadic peach-colored highlighting on a number of paragraphs in the middle.
Turned out not to be highlighting. And just removing all formatting, as some answers suggest here, was going to be a major pain (eliminating formatting will also remove all superscript and subscript, of which I had plenty).
It was "shading," fixed by going to the Paragraph options (Paint Can) and removing shading from those passages. One hitch was that I had to go paragraph by paragraph; if the section I tried to fix crossed over a paragraph break, it wouldn't work. But by
fixing a paragraph at a time, I fixed the issue without undoing all the other formatting in those passages.