Noticed the same thing starting about a week or two ago. Affects revenue flights too. I had a one-way itinerary with a free stopover per the fare rules and couldn't change just the last segment, even though I was able to successfully with the classic interface (with no fare difference) previously. Ended up cancelling and rebooking the entire reservation since the fare at the time happened to be the same, but even then it's more hassle to start a new booking from scratch.