Highlight feature cannot reliably be undone in Preview app

I enjoy the Mac’s Preview app and use it often for my graphic design business. The annotation features especially are easy to use and much more accessible than in Adobe Acrobat. (For reasons I will never understand, Adobe hides annotation features. ???) Generally, Preview works rather well, but this evening it suddenly highlighted most of the text in a document my client sent me and the undo feature would not work. I had been working on annotating the PDF file, including highlights and text annotations for about 10 minutes before Preview ‘went rogue’ on me. I found the only way to get rid of (most) of the unwanted highlighting was to reselect every text block with the highlight tool. A very cumbersome process, and Preview left funny-looking highlights at the end of each text block, which makes my work look unprofessional. I’d be grateful for any ideas on how to keep this from happening in future.