Strange issue with Alkalinity and pH

[JimK]

This is of course chem geeks territory but as you have a pebbletec/plaster finish protecting that from degradation is important. You are trying to get the pH down a little too far and maybe better shooting for pH7.5-7.7 which could decrease the outgassing of Co2 and stabilise your pH better. going lower increases the outgassing giving a bigger pH rise especially if you have a chlorinator which produces hydrogen bubbles which tend to cause the pH to drift on its own.
TA acts as a buffer to pH both in the upward drift and downward drift adding borates may help yet further as does your CYA level. but the important calcium saturation index is influenced quicker by pH than alkalinity. 80-100 is the industry standard for plaster type finishes to supply sufficient carbonates to protect the surface.

Good post.

My only experience is with our vinyl liner pool, so I don't know much about taking care of other pool types.