If its just ph/water its hard to understand. When PH drops its usually because the plant is up-taking more water then nutes and leaving the acidic nutes causing a lower PH. but a ph drop in pure water? idk man sounds odd
PH is not an issue in flushing. PH affects nute uptake only, not water. And it should go down if it is absorbing salts and unused nutes from the plant. Think about when you add nutes to water, pH goes down.
This. It's to be expected for the pH to swing a lot if you are using plain water (there's not much in the water to hold it at any specific pH and any little thing can shift the pH a dramatically). Your nutrients would have been providing the buffer in the past.
Bingo. Water by itself has no buffering capacity and thus even the CO2 in the air dissolving into the water can form carbonic acid and lower the pH quite a bit. Of course pH means absolute nothing in plain water.