That seems to be pretty typical, I have to say... I use General Hydroponics PH-Down and the same thing happens to me if, for some reason, I don't use all the water I've pH'd... so I only pH the water I'm going to use if I'm growing in soil.... I happen to grow in DWC and it takes a couple of days, when I change out my bucket, to get the pH stablized... for me, growing the way I grow (DWC), it's all a balance between nutes and water and pH down...
Good luck!