In a few screens that contain lists of elements (such as Resource Packs, Multiplayer, Language, and Statistics), by pressing Tab a few times, it's possible to highlight the list, which in turn allows you to press the arrow keys to move up or down the list entries.
However, when pressing Tab once more to move your selection away from the list, the entry highlighted previously is not unselected, which makes it indistinguishable whether or not you're still able to cycle through the list with arrow keys.
Notice that this is only the case for Resource Packs and Statistics, as the other screens mentioned here make the outline white instead of gray to indicate that the list is selected. The reason for this is likely that they have different behaviors – languages and servers can be selected by clicking, while statistics and resource packs can only be "selected" by tabbing. The bug here is that the "selected" indication is inconsistent across different screens, which should not be the case.
Confirmed in 20w48a. Relates to MC-206694