+1's not staying.
It’s a long…long time issue, with +1 turning grey, and the opposite – posts seeming to have been +1’d when you didn’t.
I guess it has decided to rear it’s head, again. I noticed it in the last few days, as well. (Posts being +1’d that I had never read before.)
A similar thing has been happening to threads, I have seen ones showing as read but I have never opened those. But that’s a different issue.