I know that the standard highlight function will not highlight a scanned document. Sometimes when I click the same highlight tool that I've pinned to the toolbar, I get a small round circle instead of the bar. The circle is like a fat marker, and I'm able to highlight freehand, anywhere on the page even if is a scanned document, and I have NOT run the text recognition. I can't find answer as to why it will do that this time but not next time. It seems random.
Please specify the application you are using.
Acrobat Pro DC