That is strange. This extension doesn't really do anything most of the time. It just hooks into the 'addNPC' path which is only triggered when a new creature is added to the combat tracker. The rest of the time it does nothing.
Perhaps the other extension you're using is also hooking into this path and there is a conflict. What is the other extension, perhaps I can take a look?
EDIT: Looking at the CombatManager code, it looks like there is another way to implement my extension that will avoid conflicts. Stay tuned...
EDIT2: I couldn't get the alternative way to work. I still don't understand why you're seeing the symptoms you're seeing, it makes no sense.