It has nothing to do with their personality, as Talkie suggested much earlier in this post. It has nothing to do with their AI settings (aggression, energy, etc.). Instead (at least in my case) it's a problem with the healers. If you have a magic class in your party who has restoration as a major skill, and if you tell them to heal (it doesn't seem to matter which of the two healing options you pick for them), then sometimes, especially in battle against more than one opponent, they stop combat and don't fight or heal. In most cases you can even initiate dialogue with them, though doing so didn't seem to make a difference for me in terms of getting them back into the fight.
I don't know if this is a shortcoming of the scripting or an actual engine limitation. It doesn't seem to matter now anyway since it looks like Talkie is finished supporting this mod. However, there are two workarounds that I found for this, neither of which I like but we seem to have no choice at this point....
1) Don't tell your companions to heal. When your companions are set to heal, they can get confused and stop fighting altogether, usually at the most inopportune time. Just having them fight but not perform any healing functions seems to keep them in the fight longer and/or indefinitely.
2) Allow one companion to heal. Your others (if you have others) will continue to fight as usual. Your healer will get confused and stop fighting at some point. When this happens, use the spell which calls your companions to your location. Once called to your location, the healer will usually get off one quick heal on you before going back to his or her confused state. You can continue to call her to you when you need a new heal.
I tried the above method in a cavern filled with very tough vampires. I've changed all the vamps in my game to be 25 levels higher than me and also gave them an ability which gives them 1200 extra health and many other advantages. They take so long to kill you go through about three weapons in the process. My party of four (including me) was massacred in several test sessions fighting a small handful of these guys at once. You see, I had three healers with me, all set to heal. When they stopped fighting a minute or so into the battle, I had to rely on health potions. When the potions ran out, I was finished.
So I tried again, but this time I used method #2 above, and it worked out much better. You see, when the NPC stops fighting, it has the sole benefit that enemeis no longer attack them (from what I could see). It's almost as if they revert to the one option in the mod where you have them avoid all fights. But since they're not being attacked, they're not getting knocked down every two seconds like the others. Which means they're usually at full health or close to it. Which means, when you need a quick heal, you summon them to your position (you might just be a few feet away---it doesn't matter, the point is casting that spell to "reset" their location). In my test of this, she healed me about 90% of the time when I cast that spell. If she failed to heal me, I'd just cast it again and she's respond properly. Doing this, I was able to defeat the cavern of uber-vamps. It took a while, but I still had health postions left at the end.
I hope this helps some people.