What you experience is an annoying behavior where keys and the tab key to navigate in the overview using the keyboard only work if the mouse pointer happens to be within one of the windows. If the cursor is outside of that area, or within two windows, the arrow keys are dead. What works, though, is to hit PgDn then PgUp again. That moves to the next workspace and back again, and once back, the arrow keys will work. So all depends on where the mouse cursor happens to be when you open the overview.
That is obviously flaky and unpredictable behavior that breaks simple keyboard navigation using arrow keys in the Activities and Application overviews. This has been there for ever, afaik. If it is not yet known to the developers, it should be reported.
You always can move the keyboard focus using the Ctrl+Alt+Tab bar, but 1) nowbody knows (or wants to know) that awkward keyboard combination 2) it is an attention requiring additional step detracting from the user's main workflow and 3) there is no reason I can see why no straightforward support for moving between items in the overview screens could be available.
It does not bother me too much because I use Alt+Tab and the search field. However, it is a major breakage for users wanting to navigate the overviews with arrow keys. This way, interacting with these screens is efficient only with the mouse or touch screen.
