I'm not sure if we are both saying the same thing but writing it differently. Yes, pack or plate the sand seam where it is going thru the dam. Your core in the dam should be doing the same thing, right? In your core, if you hit sand, you need to get down below it and sever it, then pack with clay. I still would pack the pond side of the dam too, over the sand seam.

If the vein of sand runs the whole length of the pond, it would be best to cover and pack with clay, or dig out till you hit clay again. If you hit clay again, I know your thinking it's kind of a waste of time/money because the water won't soak past that sand seam. However, if you never hit clay again(endless sand seam), then you will lose a lot of water constantly. You won't know if that sand ends or not unless you dig it, right? Another option would be to cover it with packed clay and use it as a hump in the bottom of the pond (fish structure).

These are my thoughts anyway. Perhaps others will chime in.

If you have an underground river, perhaps you could tap into that outside the perimeter of your pond and install a pump to keep the water up in your pond during droughts. It would be a nice option.