I see two upvotes on May 17 in that question's timeline.
I also only see +5 rep in your rep graph. Even though that doesn't show votes, it doesn't match the API results.
There must be a bug somewhere, but it isn't confined to the API: I can only account for the timeline/rep-graph difference through possible caching for the rep graph, and that seems unlikely. (The graph does show this isn't related to the rep cap as you didn't even get close that day.)