VNAV descent behaviour is real?

That would be my guess as well.  I like to come out of time compression TOD/-100 and start doing my wind updates descent planning etc., then save and reload as necessary around TOD/-40.

This did happen to me once in the past six months and I didn't nail down the reason, so it could be something else catawampus with the saved scenario. I can live with once a year as much as I fly her.