When a.scss imports b.scss, loading a.css indicate always the lastmodified of a event if b is more recent and has changed.
This makes the browser caching the a.css where it should asks it again when b is modified
CTRL+SHIFT+R work and the new a.css file is downloaded but still with a lastmodified of the a file while it should have the most recent.

Use case:
change the captcha system and login to admin wrongly 3 times.

To fully test it, the captcha system change should be done in production mode, since I remember that the scss import is not recursive in that mode, but the captcha scss import is at level 2 as far as I remember