A family of Microsoft word processing software products for creating web, email, and print documents.
It happens for me in "Simple Markup", the one I prefer to work in. The only mode it doesn't happen in is "All Markup", but that's an absolutely useless mode to work in when you're doing a heavy edit.
If you do decide to work in "All Markup" (as a workaround), note that you can use the settings in the Advanced Track Changes Options dialog box to choose which markup should be shown and which markup should be hidden in the "All Markup" view.