Got a picture?

For soil grows, (Sunshine 4 is in the soil category) you'll want the ingoing water to stay between 6.3 to 6.8 ph. Shoot for the 6.8 mark for a couple of waterings, and check the runoff. Speaking of which, you aren't using aquarium ph drops to check your runoff, are you? The color-coded result is skewed by the tinting in the water making the test result inaccurate.

I adjust my 8.0 wellwater to 6.8 with phDown. (phosphoric acid)

Soil buffers (lime, limestone dust, gypsum) that are added to commercial mixes fades over time. Usually lasts about 2-3 months, and your ph will start to crash. Doubtful it's your runoff ph taking a dive, more like an inaccurate testing method.