Gonna throw me 2 cents in here. I've grown in a variety of soils. PH stability is def not linear amongst them. First, I'll state that after many times of chasing my tail on what was going on with my plants, I bought an Apera 8500s Swiss glass probe direct soil PH meter a few years ago. It's accurate within 100th of a point. It wasn't cheap by any means.. almost $500. But! it did save my ass, and my whole (commercial) crop a few times, and/or helped me keep things in check.
That being said, I can say that there's not a soil on the market that's stable from start to finish. It even varies from bag to bag. When I'd go shopping for soil for a new run, I'd bring the Apera with me and stab bags till I found 30 bundles that I was content with as a starting soil. I'd see swings from the same vendor from 5.1- 7.4 I even questioned this myself and recalibrated in store. .. same results. I've found out through testing my own grow, and others, that 90% of the problem was PH related. Most of them were low. Some in the upper 4's, but the only way to really figure this out is to get a good meter, flush. For example. Let's say you get a reading in soil at 5. Your target is 6.5. You need to flush with plain water at 11.5 PH.. this is twice your reading plus where you want to be at. 5+5+1.5 =11.5, and then feed at your target pH Sounds crazy eh?.. but it works.. depending on how your medium buffers the ph. This is an experiment I've done many times. But!.. you do need a good meter to monitor your corrections. Just do the opposite for high PH... best of luck!