The pH reading when the FC level is high (> 10 ppm) won't be accurate, though usually will read falsely high if anything. As the FC level drops, the actual pH will drop as well since chlorine usage/consumpation is acidic. This is why adding bleach or chlorinating liquid has the pH rise, but then it drops back down as the FC drops.
The pH will only drop if there is something acid getting into the pool. A higher TA will have the pH rise over time due to carbon dioxide outgassing, but a lower TA will not have the pH drop though any acidic chemicals getting into the pool will have the pH drop further when the TA is lower.
Most things make the pH rise, including the curing of plaster. So I don't know the cause of your pH drop if you are using only a hypochlorite source of chlorine (e.g. bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite). If it truly continues to occur, you could raise the TA level to compensate for that though even if you had the TA high enough to keep the pH balanced, whatever acid source that's there will have you need to add more baking soda over time to keep the TA up. That is, whatever it is that is having your pH drop over time also lowers the TA as well.