Forest ponds tend to have more tannin in the water from the decay of the organics. Humic substances chemistry gets very complicated very quickly. A little bit of the forest floor could have washed in during the heavy rain events.
Did your pond water ever take on more of a "tea color" after the floods? I guess the weak acids could have used up all of your lime buffering capacity without moving your pH too far or harming your fish. (No fish kills, I presume.)
As regards the hydrilla die off - All of the experts get worried about the dissolved oxygen depletion during vegetation die offs. I have never heard of a water "chemistry" crisis from the plants that resulted in harm to the fish. (But my knowledge is very limited on that topic.)
Hopefully, one of the chemistry experts here will wander into your thread.
*** I did just remember one more thing from a college chemistry professor. He said that when you get a very strange result - run your test again!
Good luck with your RBT. You need lots of hungry ones in your pond for the Healing Waters vets! It is very cool that you do that charity.