When I had completed Tuffnut’s Win 5 battles in arena duty and claimed it the duty i got after it was the Win Streak one (Reach Win streak of 5 in Thawfest games), which was immediately completed as I already had such a win streak. I could, however, not yet claim the duty rewards as my Duty chest was completely filled up by claiming the one before it.
Now, since I still had arena energy left I went and did some arena fights. One of which I lost. To my surprise when I went to check the duty Board i saw that the progress for my Win streak duty had reset. I can understand that such a thing happens when you haven’t reached the full Win streak of 5 yet, but I find it a bit ridiculous that one that was completed also resets.