Shifting insertion point in marginal notes in MS Word for Mac

For several months, each time I open a marginal note in MS Word to edit it, I put the cursor insertion point where I want it but it moves itself to the end of the note. So I am editing away and then notice that all my work has gone to the end of the note.

This will be a non-issue to many users (and I can find no sign that anybody has posted about it anywhere on the internet) but to an editor who communicates with authors largely through marginal notes, it has actually become a significant issue. Looking forward to any insight!