Maybe it's because it's such an unusual ability--applying a positive effect of triple damage while at the same time stunning an ally, but the AI uses this ability completely backwards from the way it should.
In war, if you place Fenrir with two other monsters and only ONE of the two other monsters is immune to stun, that monster will NEVER get Rage of Fenrir used on it. Only the monster susceptible to stun will get Rage of Fenrir.
If BOTH of the other monsters are immune to stun, the ability, Rage of Fenrir, does not get used at all except very occasionally on Fenrir himself (who is not immune to stun.)
Fenrir will also use Rage of Fenrir a second time on monsters that still have triple damage turns left. IOW, Fenrir will use the ability on an ally. The triple damage lasts three turns. Then that ally won't get hit with Rage of Fenrir for a turn because of the previously stunned icon on them. Then the next turn Fenrir hits them with it again, even though they still have a turn left of the triple damage.
It's like the AI is treating allies as enemies as far as this ability is concerned. It only wants to use this ability if the stun works on an ally. The triple damage part of the ability is basically ignored by Fenrir.
I've tested this quite a bit in dungeons now, and my results have been consistent. Don't use Fenrir on defense.