If you look at your reputation tab, you can see two events for today: upvote +10, and unupvote -10. In general, the reputation tab and the reputation breakdown provide more details than the achievements dropdown.
These two events cancelled each other out - in total your reputation did not change. As far as I can tell, this is the reason why nothing related to this post was shown in the achievements dropdown.
On Meta Stack Exchange you can find several posts related to this - including some feature requests saying that some users would prefer to see the negative reputation changes, too: