Doubtful your ph numbers are accurate. FFOF is properly buffered near 6.5 to 7.0 ph, I believe. That's where it should stay for a couple of months till the buffers fade.

What are you using to check runoff ph? If it's a color-coded result, (freshwater aquarium ph test kit, ph test strips...) the tinting of the runoff will skew results.

If using a ph pen, it likely needs calibrating.

Or, you are re-using soil.

Or, they forgot the buffers in that one bag of mix.