My dad says, (old school farmer type ) that natural growth and accumulation sometimes seals porous pond bottoms.
If your pond is above the water table, I think that's true.
If the water table never reaches your pond, eventually the sand will be clogged with organic matter, silt, and finally clay to seal the deal.
If the water table rises and falls and reaches your pond water, it will be back-flushed, and may never completely seal.
