If it is like the 1/4 acre pond on our place we renovated and re-dug almost three years ago, we made a mistake in not re-coring the entire dam. We only reworked the small part we enlarged.

It had lots of trees on top, behind, and on the water side of the dam, some up to about 15 inches.
It has been leaking ever since, and likely getting a little worse with time, as the root paths give a path for leakage. We have plans to start re-coring the dam in a week or two.

There is seepage (not obvious flowing) water all along below the dam that makes a swampy area about 100 feet long and about 50 feet wide as the pond loses 1/2 to one inch per day.

If your dam had trees and roots in the dam, that is likely your answer.