Normaly it should stack. The thing you have could be a small bug. Does it only happen after you load a save game?
Yes. If I load a saved game now, it will say level 1 in hell spikes. If remove my necklace and put it back on, it will say level 2. Now if I save this game, quit, and then load this game it will say level 1 again. Kind of annoying. Have you noticed this problem?