Yes, something could be eating it.
Or now your pond is "in balance" other more aggressive/rampant species may be growing better. Nutrients in the pond are limited.
I wouldn't consider pond fertiliser, because it may lead to green water, or just to more growth of the other species.
You could just thin out the other plants a bit.

odd, I have had this stuff in my pond for years and never known it to get eaten before, even the snails dont seem to munch it.