Bob,
The last pond I had built in Dec 2016 dropped and seeped like that the whole first spring. After a summer of settling and heavy haying equipment being driven along and on the back of the dam, The seep has slowed to something I can live with, on the order of 1/8 inch per day, and some of that may be evaporation. YYMV; that didn't happen with my first pond, which was a renovation, lacking a core in the old (60+ yrs) part of the dam.