Do chemical pH adjusters cause treated water pH fluctuations?

Do chemical pH adjusters cause treated water pH fluctuations? We have some instances when Soda Ash (Sodium Carbonate) after being used to correct the pH of the treated water and the pH value measured immediately after correction kept increasing slightly as time increased at constant temperature (in the stored water). This does not happen when Caustic Soda (Sodium Hydroxide) or Hydrated lime (Calcium Hydroxide) is used.

Any explanation or advice for this? The water quality is within WHO limits.

Thanks