I saw this problem with my game so I decided to repro this (successfully) with a blank project.
All I did was create a new project, and in the blank scene add 5 buttons (allow Unity to add the EventSystem and the canvas automatically). I run the project.
Everything looks fine at first and I can click on the any of the buttons. After I do though, the 1st button gets focus. If I watch closely, it’s like something is continuously holding down the “Up arrow” key. I can fight it by holding down the “Down arrow” key, but when I let go, the ghost “Up arrow” takes over and focuses each of the buttons in turn until it goes back to the 1st one.
This is a completely blank scene and there are no other scripts running. Is this a Unity bug or am I doing something wrong?