Pond Boss Forums POND CONSTRUCTION Soil questions How would you tackle this hole?

(Go to the bottom of this post for the hole if you don't want backstory)

About Dec 2017, I had this pond dug renovated (it never held water before) by a local excavator that digs ponds, I assumed he'd do it "right" and I couldn't find any other options so I went with him ( side note, I'm amazed I can find no reputable companies around here that do this).
The biggest thing he did wrong, as pointed out by Pond Boss alumni, TJ (who is freaking awesome by the way, so much knowledge...) is he used the treads on the dozer to pack and did not use a sheepsfoot roller to pack the clay. He in fact did all of the work with the dozer.

December 2017:
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almost full by February 2018 you can see the black drain pipe on the left, I was super excited by this point.
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down ~8 feet by July 2018, much less excited.
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The pond never dried up but kept fluctuating with the rain, this is September 2019, I pumped it nearly dry, see what I discovered below the photo.


(end of backstory)

The culprit has revealed itself. I found this hole, and then one smaller about 10ft from this.
This hole is not on the dam side, it's actually on the opposite side of the pond from the dam.


I've had several contractors out to look at this, now keep in mind, these guys are like Jim-Bob from back in the hollar who just happen to do dozer work for a living, so they aren't what I would consider experts. One suggestion was rebuilding the entire dam and digging the dam down deeper, another was to go find clay somewhere nearby on my property and repacking the pond in about 2ft of clay, and yet another was to put a liner in the pond and covering the liner in clay. Someone else said dig down find the rock seam and blast it, this one sounds the most fun :p .
These all sound super expensive.

How would you suggest attempting a fix, have any of you seen a hole like this develop? what did you do?
Here are some of my cheaper ideas.
-I could fill it full of sodium bentonite + pray.
-I could pour soilfloc down in it and let the next rain carry it on into the hole (hopefully plugging things)
-Dig down with a track hoe, exposing the issue, repack it with clay -or- determine another option once I see where the water is going.