The left side listview actually gets activated when you click on it, but it just doesn't highlight the borders to indicate it.

You can test this by dragging few files on it, and then click on the left listview and use the arrow keys to select different files. It seems to be only a visual issue for me, but it doesn't affect to the actual functionality.

I also reproduced that two listviews in a vertical group don't have this issue, it's visible only when the listviews are in a horizontal group. Sounds like a MUI bug IMHO.