This is by design. It's how flexbox works. The field is meant to work in a flex-direction:column parent, where it sets its height but does not set its width (it expects to get its width from its parent). When you align it the way you do, you invert this relationships and ask the field for a width, which it reluctantly gives as its minWidth. The "stretch" option is referring to how individual elements are stretched across the row, not how single elements are stretched.