I think your analysis of this is somewhat flawed.
Technically, if you have a one line notice without an avatar then the text is top aligned in the same way as it is with an avatar, it just ends up looking vertically aligned in the middle because there's only a single line of text and the space between the top and the bottom is even.
This is behaving as designed but if you wanted to vertically align the text in the avatar case you would need to add custom CSS to achieve this.