This is not a bug. AC-sourced Subtitle labels are fluid, in that their height changes dynamically with the text inside it - so it'll only be as high as it needs to be.
If you want full control over how your UI looks, consider switching to Unity UI instead. This is now the recommended Menu display method, as AC-sourced Menus is generally more suited to rapid prototyping. If you're using the default AC Menu system, you can switch to the pre-made Subtitles UI by changing the Subtitle Menu's Source to Unity Ui Prefab.
@satoumatt: The alignment option is there because it's a standard property of all elements. Bear in mind this is only the behaviour if you Auto-update height to fit? is enabled in the label's properties box. If you uncheck it, the height will be fixed and the alignment will then be clear.
Comments
If you want full control over how your UI looks, consider switching to Unity UI instead. This is now the recommended Menu display method, as AC-sourced Menus is generally more suited to rapid prototyping. If you're using the default AC Menu system, you can switch to the pre-made Subtitles UI by changing the Subtitle Menu's Source to Unity Ui Prefab.