This is similar to bug 1601268 and is the expected behavior. There are a couple of things going on here. (1) As in bug 1601268, the tab key cycles through all results and then wraps back around to the input. In this case, there's only one result, the tip. (2) Unlike in bug 1601268, here the tip is the preselected "heuristic" result. That means two things: it's selected initially, and a selection is always present in the results. There's only one result, the tip, so it always remains selected.
Like bug 1601268, you can reproduce this before bug 1600325 landed without tips at all. For example, type "@google". There should be only a single "Search with Google" result and it should be selected. Then keep hitting the tab key.