As to how it's gone. I have about a 6' deep 1/4 acre pond. At the deepest, my pond is about 3' deep now. The only two ways that water is supposed to exit the pond is through an overflow pipe (which is full of sand) or the emergency spill off the back.

I honestly am not sure where to even start with this.

Water is most likely either evaporating or seeping out through the soil. Takes a good clay bottom to "hold" water. Sand or gravel and it will soak into ground.

This is the first year in the 6 that I've been living here that it's been an issue though. I don't suspect this year is that dramatically different from the previous years.

Feel free to reach out anytime, Seven - been helping with leaking ponds on the forum for a couple years now and am happy to chat anytime!