I'm and editor, and I've been having this issue, too. It is SOOO frustrating, and it's literally costing me money because it takes me so much longer to edit a manuscript now.
Here's what I've been doing, and it helps, somewhat.
I have to use track changes, but I like to work with 'no markup' so I can just read and edit, without the distraction of editing marks. But that view seems to make this issue horrible. So, I've been using 'All Markup,' but because it's customizable visa the dropdown menu, I hide all the changes.
This reduced the number of times that it happens, but it still happens, unfortunately.
This is obviously a track changes bug because I've tried everything to eliminate the issue. It happens in doc files and docx files. Sometimes, my paragraphs disappear, too. Or sometimes, I see double paragraphs where one paragraph is not indented but the next one is. When that happens, I save, close it out, and reopen the document, but again, it's time consuming. Time is Money, so this is horrible. I'm hoping Microsoft moderators are reading these forums and find a hotfix soon.