I have a 1/4 acre pond 11 feet deep that leaks down about 30 inches then stops or nearly stops. I have not been able to locate the source of the leak as there is water below the dam after a heavy rain regardless of the pond. The water lingers until the pond drains about 30 inches, but it is all over. Some leaks are impossible to trace.

Your leak could be coming through the dam, or under the dam, or from the bottom of the basin, then surfacing below the dam. I have determined it's almost impossible to trace a leak like that.

Shale-clay ponds around here almost always leak, with some going completely dry every year.