The property horizontalAlignBox is defined as Sets the horizontal alignment of the box's children. The box when used in grid will then just be of the size of it's content. Possible values are: left,center,right .
Therefore the element in your case “filter” just uses its css width which is unfortunatelly just covering the text “Nothing Selected” but not the full dropdown with arrows down as those two sections seem to overlap and therefore the width is just the text width.
Will take it to the product if this can be improved in future.