It seems I cannot recreate it, which is weird. The article I sent you before was one that I placed in a different category, so probably that was the reason for the change on the timestamp (I didn't know this would happen until recently), but this also happened to articles that I merely opened (in the new editor if I remember correctly) like this one. I opened it again just now both in the old version edit and the new one, but the last update didn't change this time.
I will ask the other person that also noticed that behavior to take a look and share her finds here as well.