I must contradict Peter, if there's any dust escaping then it's not good.
Festool does recommend you use it with planers and routers mostly, but that's not because the bag can't hold fine dust, which it can just fine, but because emptying the bag is a lot dirtier and thus unhealthier with fine dust, because of the dust cloud it creates. Planer and router chips on the other hand don't form a cloud which you could inhale. Emptying was never a problem for me though, I could keep my nose away from the dust cloud, or I would wear a dust mask.
The long life bag should hold all visible dust without problems. I mostly used my CT22 for sanding and sawing so it almost exclusively saw fine dust, and the LL bag worked very well, I never found any escaping dust.
Maybe you're not seating the clamp properly, try to see if there's a better way to fit it, and if that doesn't work, give Festool a call. Your LL bag definitely should not leak dust.