Hovering over the big red "+" sign indicated by the white arrow in the image does not generate a tooltip. This used to work at some point in previous versions.
The tooltips work for other effects such as when a technology gives a new item/recipe.
I am using no mods. I have checked the integrity of my game files. I am using Windows 10. It occurs across multiple saves. All other tooltips show up correctly, except this one case.
I figured it out. This is affected by the setting "Show descriptions in tooltips" under the Interface settings. I still think this is a bug if unchecking that setting simply eliminates the tooltip altogether.