I have seen in my tank the PH go up when CO2 is not sufficient and it apparently was caused by the plants consuming carbonate. As soon as the lights went off the PH started to fall. It the morning it was down to 7. But as soon as the lights came on the PH would again start to go up. Calcium carbonate is CaCO3 which has the effect of keeping the PH at about 7. But if a plant consumes the carbon it would be converted to CaOH which will strongly push up the PH.
In your pond something different is occurring . When you add a GH booster to the water you are adding calcium sulfate and magnesium sulfate . Now plants need very little sulfur but they need a lot more calcium and magnesium. The end result is the Calcium is depleted and the sulfate is left behind. Sulfate and water creates sulfuric acid and that will push the PH down. The sulfuric acid will then react with the calcium carbonate and will be converted to calcium sulfate and CO2 The same thing happens with with Calcium chloride or magnesium chloride but instead of sulfuric acid you get hydrochloric acid. And hydrochloric acid will react with carbonates the same way. It isn't KH consumption by plants or bacteria that is the problem. But instead an imbalance of mineral acids in the water Now if you are not using a GH booster in your pond your tap water will likely have a lot of sulfates and chloride in it. And as plants and bacteria consume minerals an excess of acids can still develop.
I have observed in my tanks that whenever I add my GH booster the PH of my water drops after the plants have consumed some calcium and magnesium. With a container just containing floating plants fertilizer RO water and no KH I have seen the PH drop as low as 5. As long as your water changes and GH and KH are the same after each water change the PH and KH should stay reasonably stable. But if something goes wrong and the PH falls more than normal KH can become depleted which can allow a further drop in PH. Calcium carbonate and magnesium carbonate are nice additions to a tank since a small amount can neutralize a lot of acid in a tank or pong. And it will stay as a solid. As long as it is a solid it will have minimal effect on PH, Gh, and KH test. But as soon as an acid is present they will react and dissolve which will generally cause the GH and PH to change (Both go up). KH may go up or down depending on the type of acid and how much is in the water. Injecting CO2 in a tank creates carbonic acid in an aquarium which will cause GH and kh to increase and PH to go to 7 if calcium or magnesium carbonate is present in the tank. So adding calcium or magnesium carbonate to an aquarium with CO2 injections may not be a good idea.