OK, so the back side of the dam is showing the problems?

Can you stand at the bottom of the slope below the damage and look for any soil movement? (To the right of the damage in picture #1.)

If it looks like the face of that slope has moved, then you have slumping.

If it doesn't look like any movement, then probably just a void that was poorly compacted when they were building up the back of the dam.

Either way, maybe tamp/compact some more clay in the cracks and then cover with gravel. That will also make it easier to see if it is still moving.

[It is difficult to get good compaction when there are big rocks in your mixture. Hopefully that is the cause of the problems you are observing and your core trench is doing its job correctly.]

The most important part - you said the pond appears to be holding water well? Any wet spots on the back side of the dam?

Good luck on your project! I hope you enjoy your new pond.