I was looking at my Area 51 proposal, Driving, and noticed downvotes on some of my suggested questions. I left a comment under one of the downvoted questions and immediately the score reset to zero. Looking at my reputation, it still seems to register the downvote.
Someone else just added a "test test test" comment beneath my final downvoted question, and this also reset it to zero. So it's not just the question asker who can trigger this behaviour.
This doesn't appear to be serial downvote reversal, since a commenter has added a single downvote just now and a comment from me reset the score. In fact, based on the other comments below, we may have a general voting problem on Area 51 at the moment.
(Feel free to downvote some of my proposed questions if people would like to experiment).