Mark, your photo attachment didn't show up. Could you try again as it may help identify the problem? Were the leaves actually cut as if by chewed off, as you write "cut"? Or were they loose & you could simply pull them off as a cluster of leaves or individually?
Since you've had this for 18 years, you must know how to grow it & been treating it right so excuse these questions. It wasn't overwatered at all, was it? Did anything change recently as in temperatures, light, addition of new plants around it (that might harbour some pest), or anything you can think of? Have you pulled away leaves & looked closely way down into the junction between leaves to see if any fungi or pest can be found there? I
'd be tempted to paint on a little Tanglefoot on some of the leaves where they emerge from the main cluster to see if you find any pests that only attack it at night. In case you don't know what Tanglefood it, I'm including a link. There may be similar products with diff names where you are.
Not sure if your plant won't fare worse with putting some system insecticide directly on the plant as they're meant to be absorbed through the soil. They can be very hard on some plants too, not just the insects.