"BUT later that day when the rain had stopped for nearly 8 hours I noticed the level had risen another 1""
That is not proof that there is piping into the bottom of your pond. There is always a time lag when water is running through a permeable media. A 1" water rise after 8 hours sounds reasonable.
I spit-balled a bunch of possibilities for piping problems, and it appears that they are all unlikely. You certainly have a mystery wrapped in an enigma!
I looked at your last photo again, and noticed that the collapse feature seems to be following the outline of the pond. One more possibility, is poor initial compaction by the contractor. It looks like he may have missed one strip for compaction right where his embankment ties to the edge of the undisturbed soil a few feet past the edge of your designed pond shoreline.
Hopefully, you have more rain coming. My advice, see what your pond does when it reaches full pool. If it is holding water, then I think it is unlikely that you have a significant piping problem just a few feet beyond the shoreline.
Best wishes!
