GH is a measure of calcium and magnesium.
Try to see where it is entering the tank.
Fertilizers? Do you use any root tabs? List ALL ferts.
List everything else you add to the tank.
I am especially looking for things like GH booster, Equilibrium, RO Right, Epsom Salt or anything else that has either Ca or Mg. Even some trace or micro ferts have either or both.
I also use that sort of red (or sort of brown) lava in tanks. I have a chunk that has moved from tank to tank for a LOT of years, and I have never seen it alter the water parameters. Of course this does not mean your stone is neutral. You might take out a small chunk (if you have a small one!) and test it separately in a jar or bucket of distilled water. The rock does not have to be covered in water- a large rock in a 5 gallon bucket with just a quart or so of DI would give you valid results.
Similarly, test everything else in the tank- a handful or two of substrate, any different rocks besides the lava rock, filter media... each in its separate jar with some DI to most easily show any change. Rinse off the material with DI to remove any tank water.
I am glad you have tested a couple of times, and have several types of water to test.
These should give you a range of results so you know the kit is working, and you are interpreting the results correctly.
There are some calculators out there that can help you figure out what the concentration of something will be when you are doing 50% water changes (or any other value).
Your tank is acting like it is getting 12 teaspoons (= 4 tablespoons) of Seachem Equilibrium added on top of the tap water 9 degrees.
That is a lot of change!