`, the absence of a bottom border on your button is not due to it not existing, but rather because it is not being displayed by your browser.
Your button is set to a height of 40px, and it is enclosed within a div that also has a height of 40px and a `box-sizing` property of `border-box`.
According to information found [here](https://developer.mozilla.org/en/docs/Web/CSS/box-sizing#Values), when utilizing `border-box` with `box-sizing`, the space allocated to the content (in this case, your button) is adjusted after factoring in the border and padding values of the element (the div containing your button). It appears that the content is being rendered in a way that overlaps the bottom border of the div.
Keep in mind that the `box-sizing` property in your CSS applies to all elements, including pseudo-elements like `:before` and `:after`.
To rectify the issue and display the bottom border, you can consider the following solutions:
- Decrease the height of the button element (e.g., adjust `.ms-choice` to have a height of 38px).
- Increase the height of the container div (e.g., set it to 42px), but note that this might cause misalignment with your dropdown menus and search input.
- Modify the container div to have a `box-sizing` value of `content-box`.
- Change the button's background color to transparent and apply a white background color to `div.ms-parent.form-control`. This option illustrates that the button is indeed being rendered to overlap the bottom border of the container div.