One of my pools has an issue with constant rising pH. It is a plaster pool with a SWG, built this season. I maintain this pool twice per week, Monday and Friday. It is a commercial pool so it cannot use CYA and must run at 100% to maintain FC between 3 and 5. I add about 1/2 gallon of muriatic acid almost every time, because the pH climbs to about 8.2. At first I was fighting relatively high TA, but I have since knocked the TA down to about 40. The TA has stabilized at 40, and doesn't seem to want to drop below that. Despite the stable TA, and the regular addition of acid, the pH continues to climb.
So how does the pH continue to climb with depleted TA? At some point the CO2 should run out, and the TA should drop.