A family of Microsoft word processing software products for creating web, email, and print documents.
Ahh, I see you've discovered that "Simple Markup" doesn't solve the problem. I'm an editor too, Martin, and when I'm working in "Tracked Changes" I work in Simple Markup all the time. That's when I came across this issue. I can prevent it (or stop it from happening again) by toggling to "All Markup" to make that change. Word performs as usual in "Simple Markup" for another wee while after that.
Having been paying more attention to the issue lately, it definitely happens when in the last or almost last line of a paragraph and you can anticipate it happening when you discover you can't click in the final line of the paragraph (as if that line isn't there). When I find that happening now, I switch to "All Markup", make all the edits I need to make in that paragraph, and switch back to "Simple Markup" to continue editing the document.