To reproduce, show a toast (easily done with CTRL-B, but also affects recipe/advancement toasts) and open some sort of GUI.

In container GUIs, such as chests, furnaces or droppers, the toast darkens. In the pause menu, a normal inventory, or a crafting table, the toast does not darken until a tooltip is displayed, such as by hovering over an item or a recipe.

In the creative menu, the toasts darken while showing item tooltips but not while showing tab tooltips.