The initial post is more than a little confusing, but if you're seeing a rapid drop to 3.8 pH on
Advanced Nutrients there is definitely something else wrong.
I've heard of people complaining about pH not being as stable as they think it should be, I've never had that problem and I hear from enough other people who say they never have a problem that I'm inclined to believe the instability is coming from somewhere else. But if you're seeing nearly 3pH drop there something pretty huge causing it and it's not the nutes.
The sheer fact you're seeing it in one system but not the other is proof that it's something unique to the one system. You may not think there's anything different but there has to be. When all known variables are accounted for and different results are being observed the only conclusion is that there are unseen variables that are causing the difference.
I'd recommend stripping the entire system down, rinsing the plants' roots as best you can, cleaning it relentlessly, then re-assembling and refilling it to run. Really analyze everything you do too, see if there's any conceivable difference between the way you act on one system vs the other.