Try restarting your studio to help with it or you can make it in blender as well. If it does not work when you restart your studio be sure to create a new part to union as it may be a faulty part which I have seen more and more of recently.
Because of the union bug I would just start using blender if I were you but that’s a choice of yours at the moment I do not know a way to fix it so all I can suggest is using blender. Hope you figure out the problem and have a good day.
This is a case of your unions going corrupted, so they went invisible. There’s an article in the DevForum about this.
Try exporting the union and import it as a mesh next time.