This may be a bug – not sure – unattached media not showing in grid

I've noticed that if I upload a number of images in media library and add to a media category.
I have a grid with the source as the media category.

Any images in the library that are not attached to a parent do not display in the grid.

I have to manually attach the unattached images to something in WP and then they show in the grid fine!

Multi site user, all latest versions WP, Plugins and theme