I saw the thing you posted that said it was surprisingly super low/acidic pH.
Organic soils will slowly continue to slowly acidify if they don’t have enough pH stabilizing minerals like garden lime. Sand and perlite helps stabilize pH a little.
Some nutrient mixes and soil mixes are actually counting on you watering with hard tap water, which will supply lots of “lime scale” minerals and potentially stabilize the organic acidifying process. However as you’ve also noticed in the forum, it can go the other way too and tap water can be supplying overly alkalinity to the soil or reservoir when using hard tap water.
Sometimes it feels like you just can’t win, lol.
This is why it is important to have those pH and EC/TDS/PPM testers and the knowledge of the various techniques to monitor what is happening in your soil or reservoir or even the root zone. Like measuring the run-off throughout the growing process, especially during flowering, as flowering itself, because of the way the ion uptake/exchange occurs at the root zone when the plant is happily taking up tons of nutrients during bloom and trying to create dense nuggets, will create an acidifying situation.
Hope this info helps,