I'm copyediting the document, so I don't want to see all the markup while I'm working on it and I'm not trying to print it. The red line in the margin is convenient as a visual tracker of my place in the document.

It seems switching to All Markup or toggling Track Changes off and then on again fixes the problem, but only for a few seconds to a minute, and then the line disappears again. It's ultimately more of a nuisance than an actual problem, but I'd like to know the cause.