never seen a spring lose tension over time unless they get rusty or break. At least for 30 years as the ones I set 30 years ago have never needed readjustment. However, if the diaphragm tears or leaks and the spring gets wet, it can lose tension and/or break. But normally you would notice water leaking from around the adjustment bolt and the CSV no longer functioning long before the spring got rusty or broke.
Even a slight change in pump performance should have no effect on the CSV setting. It is just not normal for the setting of the CSV to change unless someone messes with the bolt. Without the jam nut tight, just a slight bump on the adjustment bolt can make a big change as one full turn is like 14 PSI difference.
Sorry for your problem, but I don't think there is anything to fix. If it changes again let me know and we will do whatever we can to remedy the issue.