Keyboard GUI navigation priority

I noticed something strange while playing with my menu on keyboard.

I have four items arranged in a square (i.e. the top-left item should be able to navigate to the top-right and the bottom-left). If I hover over top-left, hold the right key, and then tap down, the cursor moves to the top-right and then to the bottom-right.

However, if I hold the down key and then tap right, it moves to the bottom-left and does not move to the bottom-right.

This also happens on the reverse side, so it seems as if vertical navigation is being given preference over horizontal. If a vertical key is being held, horizontal navigation is impossible.

How can I turn this off? My buttons are all set to Automatic navigation, and when I turn Visualize on, they all look like they're connected properly. I tried switching to Manual navigation but that doesn't affect the input priority issue.