I think that has to do with a sticky setting, as @bouteloua explained here. Try going to the same observation (or a different one that also has Fields) and clicking where it says Observation Fields (1). It should switch between displaying and not displaying the dropdown list.
Edit: I see that it doesn’t look quite right in your example. The arrow is pointed down rather than sideways, and the “Choose a field” box is displayed. So it may actually be a bug.