I changed its z-index in the inspector to be higher than the other control nodes, but this didn't help. I've also rearranged the scene tree twice: first, I rearranged it so that everything was in HBox and VBox containers instead of the GridContainer, and that didn't help; and second, to place the HBoxContainer that the OptionButton node is in inside of a CanvasLayer, and this didn't change anything either.
I have suspicions about what's going on (and a really sloppy way of fixing it in mind, involving importing this scene from Godot 3.5 which does work normally). But, I'm wondering if anyone has suggestions. Thanks!