Were you 100% sure the adjacent seat was occupied and not blocked ? I noticed that the distinction between the two is not possible anymore when using the app. But when you look at the seat map on the website you can still distinguish if the seat is blocked or actually occupied. This was the case yesterday for me for the two flights I took. Seat was greyed out on the app, but actually no one was there and the website showed those seats as unavailable or something like that.