I think I have seen that also (but not sure). This is from the FAQ:

"You won a game with an opponent whose BKR is significantly smaller than yours. If we would apply the formula exactly, your BKR would have to drop. But BrainKing.com uses the rule that your BKR cannot drop after you win the game, therefore it remains unchanged. The similar rule applies if you lose a game with an opponent whose BKR is significantly higher than yours"

So it's not suppose to go down. My suggestion I made earlier (where in the game over message, you list the before & after rating) - I would know for sure how my rating reacts to each win & lose.