Hi,
I want to report the same bug reported in this thread:
http://lotrminecraftmod.wikia.com/wiki/Thread:18080
It was closed because was an "optifine issue". And has been reported in the FAQ as such. But I have never installed Optifine and i have the same problem. Even I reinstalled the game and the mod, and after a while it happened on a new save.
I only use inventory tweaks apart from this mod and fastcraft.
Thanks