Well, it is the same for me too. I noticed this behavior one or two days ago, but I dismissed it thinking that maybe someone downvoted the user I just upvoted more or less at the same time.
Ok, I tried to upvote you once, it didn't stick on (a quick) reload but the arrow didn't go away. I tried to upvote and wait some 5, 10 seconds and it didn't stick and the arrow is gone!
I upvoted you, and it seems to stick. So you can upvote those below you, but not above?