Today, the following happened to me:
- I posted a question.
- I received five downvotes and one upvote on that question.
- I clicked on the achievements dropdown, seeing a −5 from that question.
- I received another upvote on that question (I checked).
The achievements alert got activated again, showing me a +5. However, when I clicked on it, the question that was the source of all of this was not shown:
(Just in case this matters, the question in question was on Meta, but it was not the one about mobile pages shown in the above screenshot.)
So, it seems that posts whose daily reputation total is zero are not shown in the achievements dropdown, even when they yielded a recent reputation gain. This is sub-optimal and confusing.