Categories in the default builds have a dynamic size if it contains more than one row of six cards. However, when creating a custom build, if a category contains more than six cards the bounding box won't resize like how it would in the default builds and instead creates a new row under and outside of the category's UI element. Only the tops of the cards in the second row are visible and interactive. This screenshot was taken in the Hideout so it is possible that it fixes itself in an actual game.
Edit: I figured out how to resize the boxes manually. Not a bug haha, sorry. I would wonder if making the resize handle a bit more obvious might be a good idea, cause it definitely took some hunting to find. Though to reiterate, not a bug
