Hummingbird broke login page CSS

I'm using Hummingbird to optimize my site, and it works very well.

However, I found out that the login page has lost all the styles: now the login form is in the upper-left corner of the page, with white background and without any styles.

The main style for the login page is login.min.css, but I can't find this file in the Hummingbird Minification page.

I excluded all CSS files from Hummingbird, but the problem persist.
If i disable the minification functionality, the login page is fine.

Is this a bug or am I missing something?