Thanks for your reply Armin, I must not have seen the notification.
I also thought about changing it to one of the used categories but this would mean it still shows up in both grids, so also the one it shouldn't with other subjects. Which would be out of my control.
Luckily!!! it seems to be fixed since the last WP update? It no longer shows in the overview, and the filter selection is identical to what I've set up in the gridder. So all good for now!