I wonder if the water level got up to the point of the topsoil and the said topsoil is porous enough for the water to escape that way.
Might shut the pump off for a week or two and see where the level stabilizes. That is what I would do. Then at least you know if the leak is somewhere lower or closer to the upper water level.
Once you know that, maybe you will have more to go on.
You say it is a "dug" pond. Was there no ravine so no dam? Or did some of the cut material be used to make a dam?
If there is a dam, check for leaks below the dam. Wet spot at base of dam or a ways below it if water coming underneath the dam. If there is no dam and some of the lower clay was not used to seal the topsoil level AND if the topsoil could lead to a nearby lower area where the water could sweep out, then the topsoil could be the problem. Look for squishy ground around the pond in a certain areas (the part that leads to a lower area where the water could go). If it is muddy/mucky then that might be where your water is going. If it is dry all the way around the pond, especially on the down hill side of the pond, then it has to be going somewhere else.
If it is the topsoil leaking and if you have any of the old clay material from the bowl area, you could lower the water to below the clay area, backhoe out a perimeter along the edge to below topsoil level, and create a clay seal around the top perimeter. Maybe would only need it on the lower side. But all this is subject to that being your problem.
Quit pumping and find the leakage area first. That would be my first step.
