Think they have been messing about with it off and on for a while now.
It does that and I've noticed this morning, that if you message a buyer from the order page, it now gives a popup box on one side of the screen, very like messenger etc.
Totally dislike it as it was far better as email, rather than messaging which they seem to want to change it too.
Though I can see how that would work better on the App, on your phone, it certainly doesn't when your on a PC!